Dextral Strike-slip Shear Research Articles

Structural analysis of the central and southwestern parts of the Neoproterozoic Salalah Crystalline Basement, Sultanate of Oman

The Salalah Crystalline Basement (SCB) is the largest Precambrian exposure in Oman located on the southern margin of the Arabian Plate at the Arabian Sea shore. It is thought to represent one of the eastern-most exposures of the Arabian-Nubian Shield. We used detailed structural analysis to establish the deformational history of the SCB by focusing on its central and southwestern parts. This part of the SCB is dominated by the Juffa complex (820 ± 10 Ma) which is made-up of two-mica gneisses, amphibolites and ultramafic lenses, and the Sadh complex (816 ± 12 Ma) which is composed of biotite and amphibolite gneisses. Our work found that the SCB evolved through four deformational events that shaped its final architecture: (1) Folding and thrusting event that resulted in the emplacement of the Sadh complex atop the Juffa complex. This event resulted in the formation of possibly N-verging nappe structure. Associated with this event was the development of mesoscopic asymmetrical tight folds and thrusts with various orientations because of folding during subsequent event. However, the dip of these structures fluctuates between SE and SW suggestive of an initial N-verging nappe structure; (2) Regional folding event around SE- and SW-plunging axes that deformed the regional fabric developed during the N-verging nappe structure. This event produced map-scale SE- and SW-plunging antiforms resulting in shaping the Juffa and Sadh complexes into a semi-dome structure; (3) Strike-slip shearing event that produced a conjugate set of NE-trending sinistral and NW-trending dextral strike-slip shear zones. The NW-trending dextral strike-slip shear zones dissected the northeastern part of the Juffa-Sadh semi-dome whereas the NE-trending sinistral strike-slip shear zones dissected the southwestern part of the semi-dome. Strong mylonitic fabric, stretching lineation and associated kinematic indicators were developed during this event; and (4) Localized SE-directed extension manifested by top-to-the-southeast kinematic indicators and the development of stretching lineation in the form of stretched amphibole crystals overprinted on older planar fabric. This is particular apparent in the hinge of the map-scale SE-plunging antiform.

Multiple Paleozoic magmatic-orogenic events in the Central Extremadura batholith (Iberian Variscan belt, Spain)

The Central Extremadura batholith located in the southeast part of the Central Iberian Zone (e.g. Iberian Autochthonous domain of the Iberian Variscan belt) was originally thought to have been generated entirely during Carboniferous igneous activity. However, some recent geochronological work has shown the existence of Ordovician plutonic rocks. The aim of this study is to re-examine the age of granitic rocks in the Central Extremadura batholith and complement this information with new field and geochemical data. This data set is used: to constrain the relative timing of plutons emplacement, as well as deformation and metamorphism preserved in the host rocks; to track deep crustal rocks and granitic magma sources; and to discuss prevailing tectonic evolutionary models for the Paleozoic evolution of the Iberian Variscan belt. We use geochemical and SHRIMP U-Pb zircon geochronology data of three granitic plutons (Ruanes, Plasenzuela and Albala) from the Central Extremadura batholith to track magmatic sources and provide a better understanding of temporal and spatial relationships between deformation and magmatism in the Iberian Variscan belt. Ruanes tonalite dated at 464 ± 2 Ma is peraluminous, magnesian and calc-alkaline, as typical of a magmatic arc setting. We report, for the first time, the occurrence of a Middle Ordovician intrusion spatially and temporally related to host deformed rocks of the Central Iberian Zone (e.g. the Iberian Autochthonous domain), which reached high-grade metamorphic conditions. Plasenzuela two-mica leucogranite is strongly peraluminous and of anatectic origin and includes a Neoproterozoic and Ordovician population of inherited zircon grains. This granite possibly derived from the partial melting of a crustal source composed of Neoproterozoic metapelites and metagreywackes (Schist-Greywacke Complex) and/or Lower Ordovician gneisses (Ollo de Sapo Formation), both including greywackes of volcano-sedimentary origin and peraluminous composition. The crystallization age of 330 ± 7 Ma obtained for the syn-kinematic Plasenzuela two-mica leucogranite constrains the functioning of D2 dextral strike-slip shear zones within the Iberian Autochthonous domain. The age of 309 ± 2 Ma obtained for the Albala cordierite-bearing monzogranite matches the age interval of the calc-alkaline magmatic suite post-dating the main Variscan D1–D3 structures in the Iberian Autochthonous domain. The new data presented in this study make it possible to recognize multiple Paleozoic magmatic-orogenic events (e.g. Caledonian, Variscan and Cimmerian) in the Central Extremadura batholith. During the Ordovician, the emplacement of intermediate magmas at shallow depths gave rise to extensive metamorphism due to heat transfer to the host rocks. The onset of this Ordovician plutonic–metamorphic complex in the Iberian Autochthonous domain is contemporaneous with the development of an active continental margin probably related to the subduction of the Iapetus–Tornquist Ocean (i.e. the Caledonian orogeny). During the Lower Carboniferous, these D2 strike-slip domains acted as lateral margins of large-scale gravitational collapses associated with the SE-direct transport of low-angle extensional shear zones (i.e. the Variscan cycle). The emplacement of Upper Carboniferous arc type granitic rocks is interpreted in the context of the amalgamation of Pangaea and the spatial proximity of Iberia relative to the Eurasian active margin in the course of Paleotethys subduction (i.e. the Cimmerian orogeny).

Kinematic analysis of secondary faults within a distributed shear-zone reveals fault linkage and increased seismic hazard

Complex multifault earthquake ruptures involving secondary faults emphasize the necessity to characterize their seismogenic potential better and study their relationship with major faults to improve the seismic hazard assessment of a region. High-resolution geophysical data were interpreted to make a detailed characterization of the Averroes Fault and the North Averroes Faults, which are poorly known secondary right-lateral strike-slip faults located in the central part of the Alboran Sea (western Mediterranean). These faults appear to have evolved since the Pliocene as part of a distributed dextral strike-slip shear zone in response to local strain engendered by the diverging movement of the Carboneras Fault to the north, and the Yusuf and Alboran Ridge faults to the south. In addition, the architecture of these faults suggests that the Averroes Fault may eventually link with the Yusuf fault, thus leading to a higher seismogenic potential. Therefore, these secondary faults represent a hitherto unrecognized seismogenic hazard since they could produce earthquakes up to moment magnitude (Mw) 7.6. Our results highlight the importance of the role played by secondary faults in a specific kinematic framework. Their reciprocal linkage and their mechanical relationship with the main faults could lead to future complex fault ruptures. This information could improve fault source and earthquake models used in seismic and tsunami hazard assessment in this and similar regions.

From an ocean floor wrench zone origin to transpressional tectonic emplacement of the Sithonia ophiolite, eastern Vardar Suture Zone, northern Greece

In the Hellenides of northern Greece, the Sithonia back-arc ophiolite constitute an element of the Vardar suture zone against the Chortiatis island arc magmatic suite, the Melissochori Formation and the Serbo-Macedonian Massif further north at the Mesozoic continental margin of Eurasia. A granodiorite from the Chortiatis island arc magmatic suite crystallized at 160 Ma as derived from new U-Pb zircon geochronology and confirms the end of arc magmatic activity that started at around 173 Ma. Located southerly of the Chortiatis island arc magmatic suite, the Sithonia ophiolite had igneous life from 159 to 149 Ma, and the ophiolite interfinger with clastic-carbonate Kimmeridgian sediments. Magmatic structures (i.e., sheeted dykes) in the ophiolite witness for NE-trending rift axis, while the transform faults and fracture zones sketch NW–SE transcurrent transtension-like propagation of the rift-spreading center at Sithonia that is consistent with a dextral wrench corridor already proposed for the ophiolite origin in the eastern Vardar zone. The tectonic emplacement of the Sithonia ophiolite involved dextral ENE to SE strike-slip sense of shear and SW and NE reverse thrust sense of shear on mostly steep foliation S1, subhorizontal lineation L1 and associated variably inclined F1 fold axes. This structural grain and kinematics are shared by adjacent Chortiatis island arc magmatic suite and the Melissochori Formation. The coexistence of strike-parallel and thrust components of displacement along discrete dextral strike-slip shear zones and internal deformation of the mentioned units is interpreted to result from a bulk dextral transpressive deformation regime developed in greenschist-facies metamorphic conditions. The back-arc ocean floor previous structural architecture with faults and fracture zones where Kimmeridgian sediments deposited in troughs was used by discrete strike-slip shear zones in which these sediments involved, and the shear zones become the sites for strain partitioning of transpressional deformation. Available biostratigraphic and radiometric age constraints define latest Jurassic-earliest Cretaceous (Tithonian-Berriasian to early Valanginian) time frame for the Sithonia ophiolite northeastward tectonic emplacement accomodated by dextral transpression that led to the ophiolite accretion to the Chortiatis island arc magmatic suite and its trench-fill exposed in the Melissochori Formation and further north toward the Serbo-Macedonian margin of Eurasia.

Strain partitioning along the Mahanadi Shear Zone: Implications for paleo-tectonics of the Eastern Ghats Province, India

During Indo-Antarctic collision at c. 1.0 Ga, Eastern Ghats Province (EGP) granulites amalgamated with the Archean Indian craton. The northern boundary of the EGP was subsequently reworked, undergoing dextral strike-slip shearing at 0.5 Ga. This study documents a phase of dextral shearing within the EGP along WNW-ESE trending shear planes in c. 0.5 Ga mylonites of the Mahanadi Shear Zone. Regional structural trends in the EGP show a swing from NE-SW to the south of the shear zone, to WNW-ESE to its north. The mylonitic shear zone foliation has a sub-horizontal lineation associated with a prominent dextral shear sense in near-horizontal sections. Electron Back Scatter Diffraction (EBSD) studies on quartz confirm that mylonitisation was associated with dextral strike-slip movement in the greenschist facies. North of the Mahanadi Shear Zone, strain was partitioned into narrow dextral strike-slip shear zones along which the older granulite fabrics were transposed parallel to later WNW-ESE trending shear planes at lower grades of metamorphism. This regional-scale shearing at ∼ 500 Ma possibly resulted in a significant dextral shift of the northern EGP with respect to the south. The shear zone was reactivated in the Permian time during deposition of Gondwana sediments in the Mahanadi basin.

Strain geometry, microstructure and metamorphism in the dextral transpressional Mubarak Shear Belt, Central Eastern Desert, Egypt

Mubarak shear belt provides an opportunity to investigate quantitative finite strain (Rs), proportions of pure shear and simple shear components, sense of shear indicators, subhorizontal to steeply plunging mineral lineations, in a dextral transpressional zone. The structural style of the Mubarak shear belt is consistent with dextral transpression within the Central Eastern Desert where dextral and reverse shear have developed simultaneously with the regional foliation. The high strain zone of the Mubarak shear belt is characterized by steeply dipping foliation with sub-horizontal stretching lineation (simple shear) surrounded by thrust imbrications with slightly plunging stretching lineations. Strain estimates from the Mubarak shear belt are used to determine how pure and simple shear components of deformation are partitioned. The axial ratios in XZ sections range from 1.16 to 2.33 with the maximum stretch, S X , ranges from 1.06 to 1.48. The minimum stretch, S Z , ranges from 0.65 to 0.92 indicating a moderate variation in vertical shortening. Volcaniclastic metasediments and metagabbros were subjected to prograde low-grade regional metamorphism in the range of greenschist to lower amphibolite facies (450–650°C at 2–4 kbar). Medium pressure (6–8 kbar at 530°C) was estimated from the high strain zone within the dextral strike-slip shear zones. Retrograde metamorphism occurred at a temperature range of 250–280°C. There is a trend towards decreasing the ratio of 100Mg/(Mg + Fetot + Mn) away from the high strain zone of the Mubarak shear belt. Integrated strain and temperature estimates indicate that the simple shear (non-coaxial) components of deformation played a significant role in formation and exhumation of the Mubarak shear belt during the accumulation of finite strain and consequently during progressive transpression and thrusting.

New structural data on Late Paleozoic tectonics in the Kyrgyz Tien Shan (Central Asian Orogenic Belt)

The tectonic evolution of the Central Asian Orogenic Belt (CAOB) is characterized by the successive accretion of lithospheric blocks, leading to different interpretations about the polarity of subductions during the Paleozoic, the number of microplates and oceanic basins and the timing of tectonic events. This is especially the case in the Tien Shan area.In this paper, we propose new structural maps and cross-sections of Middle and South Kyrgyz Tien Shan (MTS and STS respectively). These cross-sections highlight an overall dextral strike-slip shear zone in the MTS at the crustal scale and a North verging structure in the STS. These structures are Carboniferous in age and sealed by a late Carboniferous conglomerate, later overlain by Mesozoic and Cenozoic deposits. The STS exhibits two deformation phases: (1) a top-to-the-South normal shearing that can be related to subduction or exhumation dynamics and (2) a top to the North nappe stacking that we link to the late Paleozoic collisional events between the MTS and the Tarim block.We propose a new interpretation of the tectonic evolution of the Kyrgyz Tien Shan during the Late Paleozoic collision. This model involves a partitioned collisional deformation in Late Carboniferous times, with an orthogonal collision to the south, between the Tarim and MTS, and a strike-slip regime to the north along a dextral E-W zone located between the MTS and the North Tien-Shan/Kazakh platform, the so-called Nikolaev Line. The docking of the large Tarim Craton against the CAOB corresponds to a collision phase, which ended the long-lived Paleozoic subduction history in the CAOB and was followed in the TS region by intense strike-slip deformation during the Permian.

Magma‐facilitated transpressional strain partitioning within the Sawtooth metamorphic complex, Idaho: A zone accommodating Cretaceous orogen‐parallel translation in the Idaho batholith

Structural data from metasedimentary rocks and geochronologic data from intrusive rocks in the Idaho batholith provide evidence for the relationship between deformation and magmatism in the northern U.S. Cordillera. The Sawtooth metamorphic complex (SMC), Idaho, is exposed as an inlier in the central Idaho batholith and contains strongly deformed metasedimentary and intrusive rocks. Geologic mapping reveals north-south-striking, alternating contraction- and shear-dominated domains across strike. The contraction-dominated domains consist of centimeter- to tens of meter-scale, shallowly to steeply plunging upright folds with subhorizontal lineations. The shear-dominated domains are characterized by highly strained subvertical foliations, subhorizontal lineations, and syntectonic intrusive sheets. Pervasive S-C structures, winged porphyroclasts, and asymmetric folds indicate dextral strike-slip shearing. The fabrics in the two types of domains are structurally compatible and are interpreted to be broadly synchronous. This work suggests that the SMC structures represent a wrench-dominated transpressional zone, in which the regional strain partitioned into the contraction- and shear-dominated domains and the partitioning was facilitated by emplacement of syntectonic magmas. Zircon U-Pb data of the syntectonic intrusions indicate that the SMC transpressional deformation occurred mainly between ca. 95-92 Ma and ca. 84 Ma, and had ended by ca. 77 Ma. The transpressional deformation in the SMC and the western Idaho shear zone (WISZ) were kinematically compatible and partially coeval. This suggests that the SMC and WISZ represent a regional transpression system and that crustal deformation inboard of the continental margin may have contributed to the northward orogen-parallel translation of accreted terranes during the Late Cretaceous.

Kinematics of the Tengchong Terrane in SE Tibet from the late Eocene to early Miocene: Insights from coeval mid-crustal detachments and strike-slip shear zones

It is generally believed that the extrusion of SE Tibet was bounded by the dextral Gaoligong and the sinistral Ailaoshan-Red River strike-slip shear zones from the Oligocene to early Miocene. This study integrates field mapping, structural analysis and geochronology in western Yunnan (China), where foliated Precambrian basement rocks and late Cretaceous to early Eocene plutons are exposed to the west of the Gaoligong shear zone. We found that late Eocene to early Miocene flat-lying ductile shear zones were kinematically related to steeply dipping strike-slip shear zones. Four elongated gneiss domes (Donghe, Guyong, Yingjiang and Sudian) are cored by high-grade metamorphic rocks and pre-kinematic granite plutons, and bounded by top-to-NE detachments and NE-trending dextral strike-slip shear zones. Zircon U-Pb ages from LA-ICP-MS analysis and 40Ar/39Ar ages of micas and hornblende demonstrate that the flat-lying Donghe Detachment (>35–15Ma) and the Nabang dextral strike-slip shear zone (41–19Ma) were sites of prolonged, mostly coeval ductile deformation from amphibolite to greenschist facies metamorphism. The Gaoligong shear zone experienced dextral shearing under similar metamorphic conditions between 32 and 10Ma. Consistent 40Ar/39Ar ages of hornblende from the three shear zones indicate their contemporaneity at mid-crustal depth, causing the rapid exhumation and SW-ward extrusion of the Tengchong Terrane. The strain geometry and shear zone kinematics in the Tengchong Terrane are interpreted with folding of the anisotropic lithosphere around a vertical axis, i.e., the northeast corner of the Indian Plate since 41Ma. The newly discovered NE-trending Sudian, Yingjiang, and Lianghe strike-slip shear zones are subordinate ductile faults accommodating the initially rapid clockwise rotation of the Tengchong Terrane. The detachments caused mid-crustal decoupling and faster SW-ward extrusion below the sedimentary cover, whereas the strike-slip shear zones accommodated extrusion and clockwise rotation of the Tengchong Terrane around the proto-Eastern Himalayan syntaxis since the late Eocene.

A new Triassic shortening-extrusion tectonic model for Central-Eastern Asia: Structural, geochronological and paleomagnetic investigations in the Xilamulun Fault (North China)

At the northern margin of the North China Block (NCB), the Xilamulun Fault (XMF) is a key belt to decipher the tectonic evolution of Central-Eastern Asia, as it records the Paleozoic final closure of the Paleo-Asian Ocean, and localizes a Late Triassic intracontinental deformation. In this study, structural analysis, 40Ar–39Ar dating, and paleomagnetic studies were performed to investigate the kinematics of the XMF and to further discuss its Triassic geodynamic significance in the Central-Eastern Asia framework after the Paleozoic Central Asian Orogenic evolution. The structural analyses reveal two phases of ductile deformation. The first one (D1), which displays N-verging and E–W trending folds, is related to the Early Paleozoic collisional event between the NCB and the Songliao–Hunshandake Block (SHB). The second phase (D2) displays a high-angle foliation and a pervasive sub-horizontal E–W stretching lineation with kinematic criteria indicative of dextral strike–slip shearing. The 40Ar–39Ar dating on mylonitic granite places the main shearing event around 227–209 Ma. This D2 shearing is coeval with that of the dextral strike–slip Bayan Obo–Chifeng Fault (BCF) and the Chicheng–Fengning–Longhua Fault to the south, which together constitute a dextral shearing fault system on the northern margin of the NCB during the Late Triassic. The paleomagnetic study performed on the Middle Permian Guangxingyuan pluton, located between the XMF and BCF, documents a local clockwise rotation of this pluton with respect to the NCB and SHB. Our multidisciplinary study suggests an NNW–SSE shortening and strike–slip shearing dominated tectonic setting on the northern margin of the NCB during the Late Triassic. Combining the contemporaneous dextral strike–slip movements of the XMF and BCF in northern China and the sinistral strike–slip movement of East Gobi Fault (EGF) in southeastern Mongolia with the large-scale tectonic framework, a Late Triassic NNW–SSE shortening-eastward extrusion tectonic model for Central-Eastern Asia is firstly proposed. The NNW–SSE shortening results in the eastward extrusion of the continental wedge bounded by the BCF and EGF, which is accommodated by the different kinematic patterns of the southern (XMF and BCF) and northwestern (EGF) bounding faults. This shortening-extrusion tectonic framework is tentatively interpreted as the result of the far field forces associated with three Late Triassic lithosphere-scale convergences in East Asia: i) northward intracontinental subduction between the NCB and South China Block, ii) collision of the Qiangtang Block with the Qaidam Block, and iii) southward subduction of the Mongol–Okhotsk Ocean beneath the Mongolia Block.

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan, Sanandaj–Sirjan zone, Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D1. The main mylonitization event was during the D2 deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D3 folding and related mylonitization. We documented the systematic changes in the orientations of D2 linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.

Orogenic gold mineralization at the Chah Bagh deposit, Muteh gold district, Iran

The Chah Bagh gold deposit, in the Muteh gold district, is located in the central part of the Sanandaj-Sirjan zone (SSZ), Iran. Gold mineralization at Chah Bagh is hosted by a Paleozoic sequence of rocks that is dominated by deformed schist, metarhyolite, and amphibolite that exhibits a greenschist- to lower amphibolite-facies metamorphism. Three deformation events are recognized in the Chah Bagh area, D1, D2, and D3. The major NW-trending (N280–N290) dextral strike-slip shear zone in the area was formed during D2 ductile events. Gold mineralization at Chah Bagh occurred over a prolonged deformation history, but is closely related to alteration, retrograde greenschist-facies assemblages, and ductile and brittle deformation during D2 and D3. The geometry of the Au-bearing quartz veins indicates that they are temporally related to the S2 foliation and therefore to the D2 flattening and shearing. Some veins, however, are spatially and temporally related to D3 brittle normal faults and are brecciated and boudinaged during the associated shear movement. The presence of deformed Au-bearing quartz veins, and their concordant and discordant relation with respect to the main mylonitic foliation and the shear zone, indicates continuous mineralization during the D2 and D3 episodes. The Au-hosting shear zones are characterized by extensive development of heterogeneous mylonitic rocks that enhanced the permeability within the shear zones. This gave rise to further extensive dilatancy within major dilational jogs and produced a suitable structural regime for vein-hosted Au mineralization. The epigenetic Au mineralization resulted from metamorphic hydrothermal fluids circulating through major shear zones and associated structures during the late stages of orogeny. Our investigation shows that granitic intrusions have no genetic link with gold mineralization and we propose an orogenic gold model for Chah Bagh deposit, similar to Qolqoleh and Kervian in the northwestern part of the SSZ. This model is consistent with a dextral transpressional deformation kinematic model along the SSZ and, refutes the previous intrusion-related model suggested for the Muteh gold deposits. Metamorphic devolatilization and fluid flow within a ca. 30-m.y.-long period in Late Cretaceous and Early Tertiary is suggested for the genesis of the gold occurrences in both ductile and post-transpressional brittle structures.

Possible southwestward extrusion of the Ordos Block in the Late Paleoproterozoic: Constraints from kinematic and geochronologic analysis of peripheral ductile shear zones

The Paleoproterozoic is a pivotal period of the tectonic evolution of the North China Craton (NCC). Various models have been postulated by previous researchers through the study of petrology and metamorphism, but the lack of structural analyses has hampered further understanding of the tectonic evolution of the NCC. A series of well-exposed ductile shear zones developed on the northern, northwestern and eastern edges of the Ordos Block in the western part of the NCC. We carried out detailed field studies along with isotopic dating focused on the kinematics and geochronology of the ductile shear zones. On the north margin of the Ordos block, the Wulashan-Daqingshan ductile shear zone is characterized by an E-W trending shearing foliation that steeply dips 75–80° to the north or south, with subhorizontal stretching lineation plunging 10–15° toward the east or west and dextral strike-slip shearing kinematics. On the west margin of the Ordos block is the Zongbieli ductile shear zone with top-to-the-NW thrusting kinematics. On the east margin of the Ordos block is the Xueling ductile shear zone with top-to-the-SE thrusting accompanied by sinistral strike-slip shearing kinematics.The 40Ar–39Ar ages of deformed minerals from mylonitic high-grade metamorphic rocks and LA-ICP-MS U-Pb dating results of zircons from syn-tectonic anatectic granites reflect that the ductile shear zones surrounding the Ordos block deformed between 1.85 and 1.81Ga. The similar deformation ages indicate that the ductile shear zones might be genetically correlated. The geometry, kinematics and geochronology characteristics of the Wulashan-Daqingshan ductile shear zone on the north margin, the Zongbieli ductile shear zone on the west margin, and the Xueling ductile shear zone combined with the Zhujiafang sinistral ductile shear zone on the east margin defined the possible southwestward extrusion of the Ordos block in the late Paleoproterozoic, which might have resulted from the westward subduction-collision of the Eastern Block under the Western Block at approximately 1.85Ga and which is correlated with the amalgamation of the Columbia supercontinent.

Structural setting of the Young-Davidson syenite-hosted gold deposit in the Western Cadillac-Larder Lake Deformation Zone, Abitibi Greenstone Belt, Superior Province, Ontario

Within the southern Abitibi greenstone belt of the Superior Province, intrusion-related, in particular syenite-hosted gold deposits represent a significant type that is poorly understood. Whether the gold mineralization is genetically linked to the syenite intrusions (as in a magmatic model) or structurally associated with the development of subsequent shear zones remains very controversial. Our new structural data show that the multiply deformed Young-Davidson gold deposit and its surrounding Matachewan area occur along the western extension of the Cadillac – Larder Lake Deformation Zone (CLLDZ), a tectonically and economically important crustal-scale deformation zone in the southern Abitibi greenstone belt. Four generations (D1–D4) of deformation are recognized. The first part of the D1 deformation (D1a) records a NE-SW-oriented shortening and resulted in regional upright folding without penetrative foliation, whereas the subsequent D1b generates regional penetrative foliations and top-to-NE reverse faults. D2 occurs during a NW-SE-oriented shortening and produces asymmetric folds, oblique-slip thrusts and dextral strike-slip shear zones. D3 semi-brittle deformation refolds the D1 and D2 fabrics, whereas the latest D4 sinistral fault records a Paleoproterozoic brittle deformation.The Young-Davidson deposit is a lode-gold deposit that at least in part was structurally controlled. It is associated with the development of the CLLDZ and hosted in a syenite. Four main generations of veins are identified in the syenite. V1 veins are characterized by folded and boudinaged quartz–ankerite veins, V2 veins are represented by folded or en echelon quartz–pyrite veinlets, V3 veins are comprised of en echelon quartz–carbonate veins with sulfide minerals, and V4 veins consist of planar carbonate-quartz veins and minor hematite. Petrological studies reveal that the major phase of gold mineralization is associated with the V2 veins and partially with the V3 veins. Gold mineralization and emplacement of the associated veins appear to have occurred during regional D1b NE-SW-oriented shortening and top-to-NE shearing and the syenite acted as a mechanical trap due to competency contrast to the wall rock

Strain distribution within a km-scale, mid-crustal shear zone: The Kuckaus Mylonite Zone, Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D1) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D1 migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.

Combined effects of Eurasia/Sunda oblique convergence and East-Tibetan crustal flow on the active tectonics of Burma

It is widely accepted that deformation of the India/Sunda plate is the result of partitioned hyper oblique convergence. Presently, sub-meridian dextral strike slip faulting accommodates this India/Sunda motion in a buffer zone, the Burma platelet. This wide dextral strike slip shear zone is complicated by the side effect of the Tibet plateau collapse that can be described in term of crustal flow and gravity tectonics. The loss of potential energy related to this plateau collapse affects most of the Burmese platelet particularly in its northernmost part.Interaction of these two distinct geodynamic processes is recorded in the GPS based regional strain field, the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore Myanmar and Bangladesh.We propose the apparent E–W shortening component of this so called partitioned hyper-oblique subduction is only the effect of regional gravitational forces related to the Tibet plateau collapse whereas the NS strike slip faulting accommodates the India/Sunda motion.

Kinematic and thermochronological constraints on the Xincheng–Huangpi fault and Mesozoic two-phase extrusion of the Tongbai–Dabie Orogen Belt

The Tongbai–Dabie Orogenic Belt (DOB) is composed of several fault-bounded terranes with different metamorphic and deformation intensities. These tectonically stacked terranes were extruded along their boundary faults from depths of >120km during the Mesozoic. Systematic structural studies on these boundary faults can provide important information for the exhumation processes of the orogen.In this paper, we focus on the Xincheng–Huangpi fault (XHF) which is located along the southern boundary of the central DOB ultrahigh-pressure complex unit (UC) Comprehensive macro- and micro-structural, U/Pb zircon and 40Ar/39Ar geochronological data are presented here to constraint the structural chronology of the XHF. The XHF represents the lower detachment surface formed during extrusion of the UHP slab and experienced two episodes of ductile shearing. The XHF experienced early southwestward thrusting around 234Ma and was exhumed to middle crustal levels during 234–195Ma with an average exhumation rates of ca. 1.5mm/yr. The UHP rocks then exhumed to the surface by erosion. Later NW–SE trending dextral strike-slip shearing of the XHF occurred between 145 and 140Ma and was generally contemporaraneous with sinistral-oblique slip of the Xiaotian–Mozitan fault along the northern margin of the UC. Coeval dextral and sinistral-oblique shearing along the southern and northern margins of the UC would have caused southeastern lateral extrusion. This lateral extrusion has no relationship with the UHP exhumation.

Kinematics, U-Pb geochronology, and 40Ar/39Ar thermochronology of the Gold Hill shear zone, North Carolina: The Cherokee orogeny in Carolinia, Southern Appalachians

The Gold Hill shear zone is the most prominent pre-Carboniferous structure in the Southern Appalachian peri-Gondwanan tract of Carolinia. Common perception, based on indirect evidence, holds that it is an Acadian, dextral strike-slip shear zone. However, our recent structural and geochronologic studies directed at the shear zone indicate that it is a complex structure in both time and space. Structural studies in central North Carolina kinematically link deformation in the shear zone to regional shortening structures in both the hanging wall and footwall and indicate that there was a sinistral component to the deformation. Collectively, these structures constitute a regional sinistral transpressional system. We obtained nine new U-Pb zircon ages (Ediacaran–Devonian) and 12 new 40 Ar/ 39 Ar muscovite ages (Late Ordovician–Middle Mississippian); these data, in conjunction with the regional geology indicate that the shear zone has ∼12 km of stratigraphic throw and that the main motion on the zone was Late Ordovician. Collectively, geologic relations, structures, and the distribution of 40 Ar/ 39 Ar ages indicate that the shear zone was reactivated in the Late Devonian and the Middle Mississippian. The regional Late Ordovician–Silurian sinistral transpressive system, of which the Gold Hill shear zone is part, represents the most widespread tectonism documented in Carolinia; it overlaps in time with the Ordovician–Silurian Cherokee unconformity in Laurentian strata and Late Ordovician–Silurian suprasubduction-zone magmatism and metamorphism in peri-Laurentian rocks and consequently is considered a manifestation of the Southern Appalachian Cherokee orogeny, marking the accretion of Carolinia to Laurentia.

Pre-1.87 Ga development of crustal domains overprinted by 1.87 Ga transpression in the Palaeoproterozoic Skellefte district, Sweden

The complex structural evolution within the VMS-hosting Skellefte district, Sweden, has been investigated to provide a solid structural framework for the known mineral deposits in the area. The area occurs in a transition zone between dominantly N–S to NNE–WSW striking structures in the north and approximately WNW–ESE oriented structural trends in the south. The presence of high-strain zones with both the above orientations in the Skellefte district allows constraining their mutual relationship, as well as their significance for the build-up of the Svecokarelian orogen at around 1.89Ga and for the following tectonic overprint between 1.87 and 1.80Ga. The methods used in this study include structural analysis complemented by potential field modeling and SIMS U–Pb geochronology on zircon. Based on the results of this study, the earliest deformation (D1) is constrained at 1.89–1.88 (1.87)Ga and tentatively attributed to crustal extension occurring synchronously with volcanism. Deposition of the Skellefte Group metavolcanic rocks is inferred to have occurred in a pull-apart basin developed due to dextral strike-slip shearing along approximately N–S striking regional-scale shear zones. Variations in the development of deformation fabric across the district indicate that the crust was divided into an upper, un-metamorphosed domain and a lower, strongly metamorphosed domain during D1. We further infer that the transition from the upper to lower crust was locally coupled with development of low-angle crustal-scale detachment zones during D1. The heterogenous crust was subsequently overprinted by transpressional deformation which may be explained by two alternative models. According to the first model, one single SSE–NNW transpressional event with distinct strain partitioning between the coaxially deformed upper crust and the non-coaxially deformed lower crust is largely responsible for the present-day structural geometry. A post-folding rhyolite dyke, here dated at 1871±4Ma, constrains the minimum age of this event (D2). The alternative model includes two separate transpressional events: a SW–NE one at (1.88–) 1.87Ga, followed by SSE–NNW transpression at 1.86Ga. Recognition of the early-orogenic detachment zones allow us to suggest that many of the major crustal-scale shear zones in the central Fennoscandian Shield have originated as 1.89–1.87Ga crustal detachment zones, i.e. earlier than typically considered.

Polystage deformation of the Gaoligong metamorphic zone: Structures, 40Ar/39Ar mica ages, and tectonic implications

The Gaoligong metamorphic zone is located southeast of the Eastern Himalayan Syntaxis in western Yunnan, China. The zone is characterized by four stages of deformation (D1–D4). D1 structures record early compressive deformation during the Indosinian orogeny, which formed tight to isoclinal F1 folds of bedding with a penetrative S1 foliation developed parallel to fold axial planes. Mid-crustal horizontal shearing during D2 resulted in overprinting of D1 structures. D1 and D2 structures are associated with granulite facies metamorphism. D3 doming resulted in late crustal thickening and the development of a regional NW–SE trending F3 antiform. Synchronous with or slightly subsequent to D3 deformation, the zone experienced D4 ductile strike-slip shearing, resulting in its exhumation to shallow crustal levels and retrograde metamorphism. Granitic D4 mylonites predominantly yield 40Ar/39Ar mica ages of 15–16 Ma, indicating that D4 dextral strike-slip shearing occurred in the Miocene. Weakly deformed leucogranite and protomylonite yield 40Ar/39Ar ages of 10–11 Ma, suggesting that ductile strike-slip shearing continued to the Late Miocene. The new 40Ar/39Ar data indicate that escape-related deformation along the Gaoligong strike-slip shear zone occurred in the Miocene. In association with recent geophysical studies, and on the basis of the structural, crystal preferred orientation (CPO), and geochronological data presented in this paper, we suggest that the Gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of Tibet, characterized as intense deformation and metamorphism at middle–upper crustal levels.