scholarly journals 40Ar/39Ar ages (600-570 Ma) of the Serra do Azeite transtensional shear zone: evidence for syncontractional extension in the Cajati area, southern Ribeira belt

2007 ◽  
Vol 79 (4) ◽  
pp. 713-723 ◽  
Author(s):  
Rômulo Machado ◽  
Nolan M. Dehler ◽  
Paulo Vasconcelos
Keyword(s):  
Shear Zone ◽  
Dynamic Balance ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Time Interval ◽  
Alkaline Magmatism ◽  
Serra Do Mar ◽  
Continental Scale ◽  
Regional Tectonic ◽  
Basin Formation

This paper presents 40Ar/39Ar ages of the rocks from the Serra do Azeite transtensional shear zone in the southern part of the Ribeira belt, between the States of São Paulo and Paraná, and also discusses the regional correlations and the tectonic implications for other parts of the belt. The geochronological data suggest that transtensional deformation was active between 600 and 580 Ma (hornblende and muscovite apparent ages, respectively). This time span is considerably older than previous proposals for the period of activity of these structures (520-480 Ma) in the northern segment of this belt and in the Araçuaí belt. Kinematic analysis of the dated mylonites shows extensional structures with top-down movement to ESE compatible with structures found in other tectonic segments in the eastern portion of the Quadrilátero Ferrífero and in the Rio Doce Valley region. Our ages are situated in the same time interval defined for the alkaline magmatism of the Serra do Mar suite. We suggest that the regional tectonic framework was developed during continental-scale extension. This process has been coeval with convergent strain in the adjacent Neoproterozoic shear zones of the Apiaí/Ribeira and Araçuaí belts, which make up significant segments of these belts. The available data show that these structures may not be simply related to post-orogenic gravitational collapse, but must involve a more complex process probably related to dynamic balance between crustal thickening and thinning during tectonic convergence, basin formation and exhumation processes.

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans−North China orogen

2020 ◽  
Author(s):  
Lingchao He ◽  
Jian Zhang ◽  
Guochun Zhao ◽  
Changqing Yin ◽  
Jiahui Qian ◽  
...  
Keyword(s):  
Shear Zone ◽  
North China ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Slip Systems ◽  
High Grade ◽  
Ductile Shear Zone ◽  
Crustal Rocks ◽  
Ductile Shear ◽  
Lower Crustal

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans−North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650−700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans−North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans−North China orogen at 1.9−1.8 Ga.

Regional setting and kinematic features of the Needle Falls Shear Zone, Trans-Hudson Orogen

1993 ◽  
Vol 30 (7) ◽  
pp. 1338-1354 ◽  
Author(s):  
Mel R. Stauffer ◽  
John F. Lewry
Keyword(s):  
Shear Zone ◽  
Simple Shear ◽  
Country Rock ◽  
Shear Zones ◽  
Small Scale ◽  
Lake Zone ◽  
Shear Sense ◽  
Regional Tectonic ◽  
Ductile Shear ◽  
Shear Sense Indicators

Needle Falls Shear Zone is the southern part of a major northeast-trending ductile shear system within the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan. Throughout its exposed length of ~400 km, the shear zone separates reworked Archean continental crust and infolded Paleoproterozoic supracrustals of the Cree Lake Zone, to the northwest, from mainly juvenile Paleoproterozoic arc terrains and granitoid plutons of the Reindeer Zone, to the southeast. It also defines the northwest margin of the ca. 1855 Ma Wathaman Batholith, which forms the main protolith to shear zone mylonites. Although not precisely dated, available age constraints suggest that the shear zone formed between ca. 1855 and 1800 Ma, toward the end of peak thermotectonism in this part of the orogen.In the Needle Falls study area, shear zone mylonites exhibit varied, sequentially developed, ductile to brittle fabric features, including C–S fabrics, winged porphyroclasts (especially delta type), small-scale compressional and extensional microfaults ranging from thin ductile shear zones to late brittle faults, early isoclinal and sheath folds, later asymmetric folds related to compressional microfaults, and variably rotated and (or) folded quartz veins. All ductile shear-sense indicators suggest dextral displacement, as do most later ductile–brittle transition and brittle features. In conjunction with a gently north–northeast-plunging extension lineation, such data indicate oblique east-side-up dextral movement across the shear zone. However, preexisting structures in country rock protoliths rotate into the shear zone in a sense contrary to that predicted by ideal dextral simple shear, a feature thought to reflect significant flattening across the shear zone. Other ductile to brittle fabric elements in the mylonites are consistent with general noncoaxial strain, rather than ideal simple shear. Amount of displacement cannot be measured but indirect estimates suggest approximately 40 ± 20 km.The Needle Falls Shear Zone is too small and has developed too late in regional tectonic history to be considered a crustal suture. Rather, it is interpreted as either a late-tectonic oblique collisional structure or as the result of counterclockwise oroclinal rotation of the southern part of the orogen.

Crustal melt granites and migmatites along the Himalaya: melt source, segregation, transport and granite emplacement mechanisms

2009 ◽  
Vol 100 (1-2) ◽  
pp. 219-233 ◽  
Author(s):  
M. P. Searle ◽  
J. M. Cottle ◽  
M. J. Streule ◽  
D. J. Waters
Keyword(s):  
Partial Melting ◽  
Shear Zone ◽  
Internal Heat ◽  
Shear Zones ◽  
Source Rocks ◽  
Crustal Thickening ◽  
Main Central Thrust ◽  
Crustal Layer ◽  
Middle Crust ◽  
The Himalaya

ABSTRACTIndia–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

scholarly journals Crustal deformation and exhumation within the India-Eurasia oblique convergence zone: New insights from the Ailao Shan-Red River shear zone

2021 ◽  
Author(s):  
B. Zhang ◽  
S.Y. Chen ◽  
Y. Wang ◽  
P.W. Reiners ◽  
F.L. Cai ◽  
...  
Keyword(s):  
Shear Zone ◽  
Middle Miocene ◽  
Shear Zones ◽  
Strike Slip ◽  
Red River ◽  
Crustal Thickening ◽  
The Core ◽  
Early Oligocene ◽  
Lateral Extrusion ◽  
Tectonic Boundaries

During the collision of India and Eurasia, regional-scale strike-slip shear zones played a key role in accommodating lateral extrusion of blocks, block rotation, and vertical exhumation of metamorphic rocks as presented by deformation on the Ailao Shan-Red River shear zone (ARSZ) in the Eastern Himalayan Syntaxis region and western Yunnan, China. We report structural, mica Ar/Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) data from the Diancangshan massif in the middle segment of the ARSZ. These structural data reveal that the massif forms a region-scale antiform, bordered by two branches of the ARSZ along its eastern and western margins. Structural evidence for partial melting in the horizontal mylonites in the gneiss core document that the gneiss experienced a horizontal shear deformation in the middle crust. Muscovite Ar/Ar ages of 36−29 Ma from the core represent cooling ages. Muscovite Ar/Ar ages of 25 and 17 Ma from greenschist-facies mylonites along the western and southern shear zones, respectively, are interpreted as recording deformation in the ARSZ. The AFT ages, ranging from 15 to 5 Ma, represent a quiescent gap with a slow cooling/exhumation in the massif. AHe results suggest that a rapid cooling and final exhumation episode of the massif could have started before 3.2 Ma, or likely ca. 5 Ma, and continue to the present. The high-temperature horizontal shearing layers of the core were first formed across the Indochina Block, locally antiformed along the tectonic boundaries, and then cooled through the mica Ar-Ar closure temperature during Eocene or early Oligocene, subsequently reworked and further exhumed by sinistral strike-slip movement along the ARSZ during the early Oligocene (ca. 29 Ma), lasting until ca. 17 Ma, then final exhumation of the massif occurred by dextral normal faulting on the Weixi-Qiaohou and Red River faults along the limbs of the ARSZ since ca. 5 Ma. The formation of the antiform could indicate local crustal thickening in an early transpressional setting corresponding to India-Asia convergence. Large-scale sinistral ductile shear along the ARSZ in the shallow crust accommodated lateral extrusion of the Indochina Block, and further contributed to the vertical exhumation of the metamorphic massif from the late Oligocene to the middle Miocene. Furthermore, the change of kinematic reversal and associated cooling episodes along the ARSZ since the middle Miocene or early Pliocene imply a tectonic transfer from strain localization along the major tectonic boundaries to continuous deformation corresponding to plateau growth and expansion.

U/Pb geochronological constraints on Neoarchean tectonism: multiple compressional events in the northwestern Hearne Domain, Western Churchill Province, Canada

2005 ◽  
Vol 42 (1) ◽  
pp. 85-109 ◽  
Author(s):  
Kate MacLachlan ◽  
William J Davis ◽  
Carolyn Relf
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Lake Area ◽  
Thermal Event ◽  
Grade Metamorphism ◽  
Geochronological Constraints

In the Yathkyed Lake area the Tyrrell shear zone (TSZ) marks the boundary between the northwestern and central Hearne subdomains of the Western Churchill Province. The TSZ is dominated by Proterozoic dextral strain, but in areas of low dextral strain, older shear fabrics are consistent with an earlier thrusting event. Four syntectonic granite sheets from within low-dextral-strain zones in the TSZ and contiguous shear zones at the base of the Yathkyed belt have ages of 2636–2629, 2652 ± 8, 2665–2639, and 2644 ± 3 Ma. The Yathkyed belt occurs in the hanging wall of these shear zones and comprises a greenschist- to amphibolite-grade, overturned panel that is overlain by an upper amphibolite-grade polydeformed panel. The geometry is consistent with a thrust-imbricated stack. Two episodes of deformation in the hanging wall are bracket between ca. 2660 and 2616 +6–4 Ma and between 2616 +6–4 Ma and ca. 2.60 Ga. The Yathkyed belt is interpreted to have initially been emplaced as a thick-skinned, thrust nappe along the TSZ, during 2.66–2.62 Ga tectonism, and reactivated at ca. 2.62–2.60 Ga, causing further crustal thickening. In the Upper Panel, protracted upper amphibolite-grade metamorphism and associated anatectic melting are recorded by zircon, titanite, and monazite ages ranging from ca. 2561 to 2492 Ma. This thermal event is in part attributed to burial in response to ca. 2.62–2.60 Ga thrust reactivation. The timing and means of emplacement of the Upper Panel on top of the Lower Panel is uncertain; however, ca. 2.50 Ga thrusting elsewhere in the northwestern Hearne subdomain provides one possible mechanism.

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

Lithosphere ◽  
2021 ◽  
Vol 2021 (Special 6) ◽  
Author(s):  
Sergio P. Neves ◽  
Andréa Tommasi ◽  
Alain Vauchez ◽  
Thais Andressa Carrino
Keyword(s):  
Shear Zone ◽  
Large Scale ◽  
Shear Zones ◽  
Strike Slip ◽  
Northeastern Brazil ◽  
Main Stage ◽  
Plate Boundaries ◽  
Deformation Localization ◽  
Continental Scale ◽  
Dextral Shear

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

scholarly journals The Capo Castello Shear Zone (Eastern Elba Island): Deformation at the Contact between Oceanic and Continent Tectonic Units

Geosciences ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 361
Author(s):  
Caterina Bianco
Keyword(s):  
Shear Zone ◽  
Strain Localization ◽  
Shear Zones ◽  
Mature Stage ◽  
Low Grade ◽  
Strain Partitioning ◽  
Initial Stage ◽  
Regional Tectonic ◽  
Elba Island ◽  
Tectonic Contact

Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.

scholarly journals Migmatite and leucogranite in a continental-scale exhumed strike-slip shear zone: Implications for tectonic evolution and initiation of shearing

2021 ◽  
Author(s):  
Junyu Li ◽  
Shuyun Cao ◽  
Xuemei Cheng ◽  
Franz Neubauer ◽  
Haobo Wang ◽  
...  
Keyword(s):  
Shear Zone ◽  
Thermal State ◽  
Shear Zones ◽  
Strike Slip ◽  
Red River ◽  
Crustal Anatexis ◽  
Plate Convergence ◽  
Continental Scale ◽  
Geological Processes ◽  
Lower Crustal

Plutons within continental strike-slip shear zones bear important geological processes on late-stage plate transpression and continent-continent collision and associated lateral block extrusion. Where, when, and how intrusions and shearing along transpressional strike-slip shear zones respond to plate interactions, however, are often debated. In this study, we investigated migmatite associated leucogranite and pegmatite from the exhumed >1000-km-long Ailao Shan-Red River left-lateral strike-slip shear zone in Southeast Asia that was active during India-Eurasia plate convergence. Most zircons from the migmatites and leucogranitic intrusions present inherited core-rim structure. The depletion of rare earth element patterns and positive Eu anomalies suggest that leucosomes and leucogranites are the result of crustal anatexis. Zircon rims from the foliated migmatites and leucogranites record U-Pb ages of 41−28 Ma, revealing the timing of the Cenozoic crustal anatexis event along this strike-slip shear zone. Ages of the magmatic zircons from the unfoliated pegmatites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 26−23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red River strike-slip shear zone indicates that thickened crust underneath the shear zone involved previously subducted crust. We propose that the Cenozoic thermal state has an important effect on the crustal anatexis and thus on the rheological behavior of the lithosphere by thermal weakening, which plays an essential role in localizing the initiation of the deep-seated lower-crustal shear zone.

scholarly journals Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

2018 ◽  
Vol 55 (9) ◽  
pp. 1063-1078 ◽  
Author(s):  
Michelle J. Markley ◽  
Steven R. Dunn ◽  
Michael J. Jercinovic ◽  
William H. Peck ◽  
Michael L. Williams
Keyword(s):  
Shear Zone ◽  
Crustal Thickening ◽  
Boundary Zone ◽  
Grenville Province ◽  
Metamorphic History ◽  
Crustal Thinning ◽  
Tectonic Significance ◽  
History Of ◽  
Central Gneiss Belt ◽  
Scale Shear

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

scholarly journals The Dabie Extensional Tectonic System: Structural Framework

2012 ◽  
Vol 2012 ◽  
pp. 1-8 ◽  
Author(s):  
Quanlin Hou ◽  
Hongyuan Zhang ◽  
Qing Liu ◽  
Jun Li ◽  
Yudong Wu
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Ultrahigh Pressure ◽  
Metamorphic Complex ◽  
The South ◽  
Magma Intrusion ◽  
The North ◽  
Extensional Tectonic ◽  
Mechanism Transition ◽  
Tectonic System

A previous study of the Dabie area has been supposed that a strong extensional event happened between the Yangtze and North China blocks. The entire extensional system is divided into the Northern Dabie metamorphic complex belt and the south extensional tectonic System according to geological and geochemical characteristics in our study. The Xiaotian-Mozitan shear zone in the north boundary of the north system is a thrust detachment, showing upper block sliding to the NNE, with a displacement of more than 56 km. However, in the south system, the shearing direction along the Shuihou-Wuhe and Taihu-Mamiao shear zones is tending towards SSE, whereas that along the Susong-Qingshuihe shear zone tending towards SW, with a displacement of about 12 km. Flinn index results of both the north and south extensional systems indicate that there is a shear mechanism transition from pure to simple, implying that the extensional event in the south tectonic system could be related to a magma intrusion in the Northern Dabie metamorphic complex belt. Two 40Ar-39Ar ages of mylonite rocks in the above mentioned shear zones yielded, separately, ~190 Ma and ~124 Ma, referring to a cooling age of ultrahigh-pressure rocks and an extensional era later.

Sign in / Sign up

Export Citation Format

Share Document