Deep-crustal deformation textures along megathrusts from Newfoundland and Ontario: implications for microstructural preservation, strain rates, and strength of the lithosphere

1992 ◽  
Vol 29 (2) ◽  
pp. 328-337 ◽  
Author(s):  
Joseph Clancy White ◽  
Christopher K. Mawer
Keyword(s):  
Shear Zone ◽  
Upper Mantle ◽  
Shear Zones ◽  
Threshold Temperature ◽  
Strain Rates ◽  
Grenville Province ◽  
Preservation Condition ◽  
Deformation Textures ◽  
Highly Strained ◽  
Lower Crustal

Lithospheric-scale thrusts from the west Newfoundland ophiolite belt (White Hills Peridotite shear zone) and the south-western Grenville Province (Parry Sound shear zone) involve rocks of lower crustal and (or) upper mantle origin that exhibit intense crystal-plastic deformation of plagioclase, K-feldspar, orthopyroxene, and clinopyroxene, minerals that are commonly viewed as representative of low-ductility phases. The occurrence of this extreme deformation in shear zones that exhibit similar lower crustal syntectonic P–T conditions suggests a phenomenological link between the megathrust environment and both the generation and subsequent preservation of the observed deformation microstructures. An empirical homologous parameter is constructed in an attempt to characterize conditions for similar behaviour among different minerals and to explore the feasibility of refining a threshold recovery–preservation condition within the megathrusts studied. This parameter predicts, at the estimated syntectonic temperature of 800 °C, the similarity of microstructures in highly strained albite and orthopyroxene crystals observed in both megathrusts. This temperature is interpreted as a lower limit for the upper threshold of microstructure preservation in albite and orthopyroxene for the particular megathrust history. Comparison of tectonic constraints with strain rates calculated at the inferred threshold temperature for several minerals with tectonic constraints indicates that strain rates of at least 10−12 s−1 are both rheologically possible and geometrically plausible in shear zones of kilometre-scale widths. The associated lithosphere strength during megathrust displacement is on the order of 1–50 MPa. These data support formation of synkinematic records within shear zones that preserve evidence of lithospheric behaviour over crustal-thickness length scales.

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.

scholarly journals Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy

Minerals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1036
Author(s):  
Jolien Linckens ◽  
Sören Tholen
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Upper Mantle ◽  
Microstructural Analysis ◽  
Shear Zones ◽  
Geodynamic Setting ◽  
Mixing Processes ◽  
Phase Mixing ◽  
Fine Grained ◽  
Order Of Magnitude

Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

A test of detailed Nd isotope mapping in the Grenville Province: delineating a duplex thrust sheet in the Kipawa–Mattawa region

2006 ◽  
Vol 43 (4) ◽  
pp. 421-432 ◽  
Author(s):  
M K Herrell ◽  
A P Dickin ◽  
W A Morris
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Data Sets ◽  
Aeromagnetic Data ◽  
Thrust Sheet ◽  
Grenville Province ◽  
Nd Isotope ◽  
Tectonic Model ◽  
Thrust Sheets ◽  
Archean Crust

Over sixty new neodymium model ages were determined on orthogneisses from the Kipawa–Mattawa region of the Grenville Province to refine previous Nd isotope mapping work in this area. The combined Nd data sets support a tectonic model involving three major thrust sheets in the Kipawa area, separated by major shear zones. The uppermost sheet is correlated with the Allochthonous Polycyclic Belt, represented in the study area by the Lac Watson nappe, along with two allochthonous klippen. These have Nd model ages < 1.8 Ga, consistent with previous work. Within the underlying Parautochthonous Belt, previous workers identified a second major shear zone, separating rocks with Archean and Proterozoic crystallization ages, respectively. These two thrust sheets also have distinct Nd isotope signatures. The lowermost sheet consists of metamorphosed but otherwise relatively pristine Archean crust with Nd model ages > 2.6 Ga, whereas the overlying sheet consists of magmatically reworked Archean parautochthon with model ages from 1.8–2.6 Ga. A residual magnetic-field map developed from aeromagnetic data was compared with the terrane boundaries determined from isotopic data. The aeromagnetic data accurately reflect the margin of relatively pristine Archean crust in the study area, but this boundary does not correspond to the Allochthon Boundary Thrust. Instead, this boundary resulted from downcutting of the basal shear zone of the allochthon. This caused décollement of the strongly reworked Archean parautochthon to generate a duplex thrust sheet that was transported northwestwards over pristine Archean crust.

Metagabbronorite from DSDP hole 334: an example of high-temperature deformation and recrystallization near the Mid-Atlantic Ridge

1977 ◽  
Vol 14 (4) ◽  
pp. 886-898 ◽  
Author(s):  
Herwart Helmstaedt ◽  
John M. Allen
Keyword(s):  
Shear Zone ◽  
Regional Metamorphism ◽  
Shear Zones ◽  
Granulite Facies ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Mid Atlantic Ridge ◽  
Magmatic History ◽  
Highly Strained ◽  
Zoned Plagioclase

Five gabbros and three peridotites from hole 334 were examined for postmagmatic deformational changes and metamorphic recrystallization. The condition of the gabbros ranges from unaltered, to slightly altered and deformed, to highly deformed and metamorphosed, the latter type being confined to narrow shear zones. Petrofabric studies and electron microprobe analyses of mineral phases of a foliated metagabbronorite from such a shear zone suggest the following post-magmatic history: (1) strong plastic deformation and anhydrous synkinematic recrystallization of igneous plagioclase (An90−85), clinopyroxene, and orthopyroxene to the granulite facies mineral assemblage of strongly zoned plagioclase (An80−65), diopside, and orthopyroxene; relics of the igneous minerals survived as highly strained porphyroclasts; (2) synkinematic growth of amphibole consisting of a patchy intergrowth of tremolite and actinolitic hornblende; the amphibole fabric is compatible with strain estimates by the centre-point method; (3) mainly static hydrous alteration — growth of (?) vermiculite and sericitization.Penetrative deformation probably initiated shortly after intrusion and took place during cooling of the rocks from approximately 800 °C to 300 °C. The bulk of the strain in the metagabbronorite of the shear zone predates the serpentinization of the peridotites. Many textural and fabric features in alpine-type gabbro–peridotite complexes that are ascribed to regional metamorphism and deformation during orogenic emplacement may originate shortly after intrusion near or at an accreting plate margin.

Combined oxygen isotope – compositional studies of some granitoids from the Grenville Province of Ontario, Canada: implications for source regions

1986 ◽  
Vol 23 (9) ◽  
pp. 1412-1432 ◽  
Author(s):  
Tsai-Way Wu ◽  
Robert Kerrich
Keyword(s):  
Rare Earth ◽  
Oxygen Isotope ◽  
Rare Earth Element ◽  
Fractional Crystallization ◽  
Upper Mantle ◽  
Earth Element ◽  
Thermal Waters ◽  
Grenville Province ◽  
Source Regions ◽  
Lower Crustal

Oxygen isotopic compositions of whole rocks and coexisting quartz–feldspar pairs have been determined for nine pre-, and syn- to late-kinematic granitoid plutons in the Grenville Province of Ontario. These new data demonstrate that granitoid rocks (Algonquin, Mulock) in migmatite terrain of the Ontario Gneiss Segment possess normal δ18O values (<9.0‰), whereas mesozonal to epizonal plutons (Elphin, Coe Hill, Deloro, Barber's Lake) in the Central Metasedimentary Belt (CMB) are characterized by significantly higher 18O contents (δ18O > 9.0‰), in accord with previous results.In the Algonquin sodic suite, a gross covariance of δ18O with compositional indices is present, from 6.4‰, SiO2 = 50.5 wt. % (gabbro) to 8.7‰, SiO2 = 72 wt. % (trondhjemite), resulting from combined assimilation–fractional crystallization. Mafic members of the sodic suite are 18O enriched overall (5.8–7.9‰) relative to fresh tholeiites (5.7 + 0.3‰), implicating some 18O contamination of the protolith. The dispersion of δ18O values in the Algonquin potassic suite, from 4.3 to 9.3‰, is independent of composition and attributed to isotopic exchange with low-18O thermal waters during emplacement. Biotite–hornblende granite of the Mulock batholith is characterized by a limited oxygen isotope compositional range, where the average δ18O = 8.1 ± 0.5‰; δ18O correlates with SiO2 but not with the zonal distribution of Ba, Rb, and Sr abundances.The Union Lake quartz diorite (δ18O = 8.5 ± 0.1‰) and White Lake trondhjemite (δ18O = 7.3 ± 0.6‰) have oxygen isotope compositions comparable to those of other trondhjemitic suites in the CMB. A systematic enrichment of ~1.2‰ in the Union Lake pluton, together with enhanced Ca, Mg, Fe, and Sr, can be accounted for by assimilation of ~5% marbles and 10% amphibolites from the country rock. Uniformly high δ18O values of 11.5 ± 0.8‰ characterize the Elphin granite–syenite complex. The largest values (11.7–12.7‰) and lowest SiO2 (54–56 wt. %) are in the partially assimilated host gabbro–diorite complex, endorsing the presence of 18O-enriched source regions. The Cheddar biotite–hornblende granite, one of a population of intrusions within the alkalic belt of the western CMB, has a restricted isotopic span, where δ18O = 8.8 ± 0.9‰. An unusual concave rare-earth-element (REE) distribution may result from interaction with a heavy rare-earth -element (HREE) enriched volatile phase. The Coe Hill biotite granite (δ18O = 10.4 ± 0.4‰) is isotopically in compliance with other granites and syenites of the CMB. Covariance of δ18O and SiO2, in conjunction with smooth and continuous geochemical trends, is interpreted in terms of assimilation–fractional crystallization.Peralkaline granite of the Deloro pluton includes a hypersolvus phase with high, scattered δ18O values (9.1–11.8‰) and a subsolvus counterpart attributed to late influx of water that induced isotopic reequilibration toward a more constrained range (δ18O = 9.2–10.2‰). REE distributions of a calcic syenite phase are compatible with its evolution by fractional crystallization of a low-K tholeiitic magma, and the high-18O character (δ18O = 11.1–12.6‰) requires 18O enrichment of the protolith and (or) 18O contamination of the magma. Peralkaline rhyolitic volcanics, compositionally coherent with the Deloro pluton and possibly representing extrusive equivalents, possess significantly higher and more variable δ18O values, from 11.7 to 14.2‰; this is attributed to 18O enrichment during low-temperature exchange with thermal waters, superimposed on a primary high-18O magma. The Barber's Lake two-mica granite contains enhanced abundances of U (15 ppm) and Th (36 ppm) in conjunction with systematically elevated δ18O values (10.4 ± 0.5‰). Geochemical constraints are compatible with its evolution from a trondhjemitic magma, but the isotopically enriched nature requires extensive 18O contamination of the protolith and (or) magma. These nine granites variously retain "memory" of primary and (or) secondary features, including δ18O of the source region, covariance of isotopic and compositional parameters, and sporadically superimposed disturbance by exchange with thermal waters. During metamorphism, quartz and feldspar were systematically reset to high-temperature fractionations, but the extent of open-system exchange with rock reservoirs was limited.Despite some probable disturbance by metamorphism and the limited data available, O–Sr isotope systematics of the Grenville granitoids indicate that (1) high-18O granites from the Frontenac Axis were derived from in situ anatexis of Grenville Supergroup metasediments, (2) synkinematic granites were derived by mixing of a primary magma generated at a lower crustal (granulite facies) or upper mantle level with the fusion products generated by partial melting of the Archean–Early Proterozoic type metasediments, and (3) the tonalite–trondhjemite suite in this part of the Grenville Province was derived from a similar lower crustal or upper mantle primary magma by direct fractional crystallization.

Crustal shear zones and thrust belts: their geometry and continuity in Central Africa

1986 ◽  
Vol 317 (1539) ◽  
pp. 111-128 ◽  
Keyword(s):  
Shear Zone ◽  
Central Africa ◽  
Shear Zones ◽  
Strike Slip ◽  
Thrust Belts ◽  
Ductile Shear Zones ◽  
Displacement Direction ◽  
Crustal Shear Zones ◽  
Orogenic Belts ◽  
Lower Crustal

The Precambrian orogenic belts of Africa are often defined by ductile shear zones which developed in response to large displacements, and which mark orogenic ‘ fronts ’ between mobile and stable parts of the crust. They are thought to represent the major crustal reflectors seen by seismic reflection profiling in younger orogenic belts. These orogenic fronts are connected by shear zones that transfer displacement or accommodate different displacements, between orogenic segments. Smaller shears within an orogenic belt occur as a result of differential movements. These shear zones are seen to pass from flat-lying to steep structures and may have a thrust or strike-slip sense. They compare with the staircase trajectories characteristic of foreland thrust belts. In common with thrust belts, the geometry of the shear zones can be used to estimate displacement direction, as can regional extensional fabrics developed in the associated high-strain tectonites. Central Africa has been previously described as a complex network of late Proterozoic ‘mobile belts’. The recognition of similar displacements and time equivalence in these belts allows their reinterpretation in terms of a linked thrust and strike-slip shear-zone system. An example is the Damaran, Lufilian, Zambezi and Ukingan system. These orogenic belts share a similar displacement picture and broad time equivalence and were apparently linked in a lower crustal shear zone of continental dimensions. This shear zone system appears to have developed under a single tectonic framework

The Ailao Shan–Red River shear zone revisited: Timing and tectonic implications

2019 ◽  
Vol 132 (5-6) ◽  
pp. 1165-1182 ◽  
Author(s):  
Junlai Liu ◽  
Xiaoyu Chen ◽  
Yuan Tang ◽  
Zhijie Song ◽  
Wei Wang
Keyword(s):  
Shear Zone ◽  
Structural Data ◽  
Shear Zones ◽  
Red River ◽  
Plate Convergence ◽  
Ductile Shearing ◽  
Magma Sources ◽  
T Values ◽  
Tectonic Extrusion ◽  
Lower Crustal

Abstract Continental strike-slip shear zones that may bear important information about the evolution of convergent tectonics often occur to accommodate plate convergence. When and how shearing along the shear zones responds to plate interactions, however, are often debated. In this study, we investigated the Oligocene–Miocene leucocratic dikes from the Ailao Shan–Red River shear zone, which was active during India-Eurasia plate convergence, to constrain the timing and mechanism of ductile shearing along the shear zone. The dikes are structurally grouped into pre-, syn-, and postkinematic types with respect to ductile shearing. Prekinematic dikes from ca. 41 to 30 Ma have low whole-rock 87Sr/86Sr(i) values (0.707–0.710), generally high εNd(t) values (–3.31∼–7.98), and variable εHf(t) values (–7.9∼+5.7). Their magma sources involved high thermal perturbation inducing partial melting of the lower crust, and contributions from the mantle that were possibly related to extensional collapse of the orogenic belt prior to tectonic extrusion of the Sundaland block. Syn- and postkinematic dikes from ca. 28 to 20 Ma dominantly have high whole-rock 87Sr/86Sr(i) (0.707–0.725) and low εNd(t) (–5.83 to –9.76) values, and either negative or positive zircon εHf(t) values (broadly in the range of –12 to + 7.6) for coeval but separate crustal magma sources. The results imply that major shearing accompanying retrograde metamorphism along the Ailao Shan–Red River shear zone was localized to crustal level. A synthesis of regional structural data suggests that Oligocene–Miocene shearing along the Ailao Shan–Red River shear zone and lateral tectonic extrusion of the Sundaland block proceeded in response to progressive India-Eurasia plate convergence. Distributed and inhomogeneous middle- to lower-crustal flow along the boundaries of and within the Sundaland block occurred during the tectonic extrusion.

Geochronological and kinematic constraints on crustal shortening and escape in a two-sided oblique-slip collisional and magmatic orogen, Paleoproterozoic Taltson magmatic zone, northeastern Alberta

2000 ◽  
Vol 37 (11) ◽  
pp. 1549-1573 ◽  
Author(s):  
Michael R McDonough ◽  
Vicki J McNicoll ◽  
Ernst M Schetselaar ◽  
Timothy W Grover
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Remotely Sensed Data ◽  
Kinematic Data ◽  
Magmatic Arc ◽  
Magmatic Rocks ◽  
Ductile Shear Zones ◽  
Continental Magmatic Arc ◽  
Northeastern Alberta ◽  
Lower Crustal

The southern Taltson magmatic zone (south of 60°N) is a composite continental magmatic arc and collisional orogen resulting from the convergence of the Buffalo Head terrane with the Archean Churchill craton. Taltson basement (ca. 3.2–3.0 Ga and 2.4–2.14 Ga) and Rutledge River supracrustal gneisses (2.13–2.09 Ga) were intruded by voluminous I- and S-type magmatic rocks between 1.99 and 1.92 Ga. Taltson magmatic zone was deformed by three ductile shear zones: Leland Lakes, Charles Lake, and Andrew Lake, exhibiting both strike- and dip-lineated mylonitic domains. Kinematic data for shear zones are reported at microscopic, mesoscopic, and macroscopic (remotely sensed data) scale. We present field and U–Pb isotopic data (zircon and monazite) for magmatic and metamorphic rocks that constrain the timing of granulite to upper amphibolite-grade shearing in the Leland Lakes and Charles Lake (formerly Allan) shear zones to ca. 1938–1934 Ma. Foreland (easterly) vergent thrusting on the Andrew Lake shear zone is ca. 1932 Ma. Taltson shear zones were overprinted by widespread amphibolite- to greenschist-grade shearing, which is constrained by published 40Ar–39Ar and K–Ar dates on hornblende and muscovite to between ca. 1900 and 1800 Ma. We propose a crustal architecture, resembling a crustal-scale asymmetric flower structure, in which the Charles Lakes shear zone formed the fundamental shear zone of a middle to lower crustal sinistral transpression system that accommodated southward escape of crust in the upper plate of an oblique continental subduction–collision zone, with shortening partitioned into synchronous outwardly vergent thrust systems to the east and west of the main shear zone.

scholarly journals Olivine-induced viscous anisotropy in fossil strike-slip mantle shear zones and associated strain localization in the crust

2020 ◽  
Vol 224 (1) ◽  
pp. 608-625
Author(s):  
Lucan Mameri ◽  
Andréa Tommasi ◽  
Javier Signorelli ◽  
Riad Hassani
Keyword(s):  
Boundary Conditions ◽  
Shear Zone ◽  
Strain Localization ◽  
Strain Distribution ◽  
Seismic Anisotropy ◽  
Shear Zones ◽  
Strike Slip ◽  
Intraplate Seismicity ◽  
Strain Rates ◽  
Mechanical Coupling

SUMMARY We propose that strain localization in plate interiors, such as linear belts of intraplate seismicity, may arise from spatial variations in viscous anisotropy produced by preferred orientation of olivine crystals (CPO or texture) inherited from previous deformation episodes in the lithospheric mantle. To quantify this effect, we model the deformation of a plate containing a fossil strike-slip mantle shear zone at different orientations relative to an imposed horizontal shortening, but no initial heterogeneity in the crust. The fossil shear zone is characterized by different orientation and intensity of the olivine CPO relatively to the surrounding mantle, which is isotropic in most simulations. The anisotropy in viscosity produced by the CPO, which remains fixed throughout the simulations, is described by an anisotropic (Hill) yield function parametrized based on second-order viscoplastic self-consistent (SO-VPSC) models. The results indicate that lateral variations in viscous anisotropy in the mantle affect the strain distribution in the entire lithosphere. Reactivation of the strike-slip mantle shear zone and strain localization in the crust above it occur for horizontal compression at 35–55° to the fossil shear plane, with a maximum at 45°. The magnitude of strain localization depends on (i) the contrast in viscous anisotropy and, hence, on the variations in CPO orientation and intensity in the mantle, (ii) the boundary conditions and (iii) the feedbacks between mantle and crustal deformation. For a strong olivine CPO, when the boundary conditions do not hinder shear parallel to the fossil mantle shear zone, strain rates within it are up to a factor 30 higher than in an isotropic surrounding mantle or up to a factor 200 when the surrounding mantle is anisotropic, which results in strain rates up to a factor 10 or up to a factor 100 higher in the crust right above the fossil shear zone. Frictional weakening in the crust faults increases strain localization in the entire lithospheric column. High strength contrasts between the mantle and the ductile crust result in less efficient mechanical coupling, with strong localization in the mantle and lower crust, but weak in the brittle upper crust. Decrease in the crust–mantle strength contrast enhances the coupling and produces more homogenous strain distribution with depth, as well as a time-dependent evolution of strain localization, which reaches a peak and decreases before attaining steady-state. Comparison of seismic anisotropy, regional stress and focal mechanism data in linear arrays of intraplate seismicity, like the New Madrid and South Armorican seismic zones, to our models' predictions corroborates that olivine CPO preserved in fossil lithospheric-scale shear zones may be key for the development of such structures.

The Nusfjord exhumed earthquake source (Lofoten, Norway): deep crustal seismicity driven by bending of the lower plate during continental collision

2020 ◽  
Author(s):  
Luca Menegon ◽  
Lucy Campbell ◽  
Åke Fagereng ◽  
Giorgio Pennacchioni
Keyword(s):  
Shear Zone ◽  
Lower Crust ◽  
Tectonic Setting ◽  
Shear Zones ◽  
Continental Collision ◽  
Earthquake Source ◽  
Ductile Deformation ◽  
Lower Plate ◽  
Caledonian Orogeny ◽  
Lower Crustal

&lt;p&gt;&lt;span&gt;&lt;span&gt;The origin of earthquakes in the lower crust at depth of 20-40 km, where dominantly ductile deformation is expected, is highly debated. Exhumed networks of lower crustal coeval pseudotachylytes (quenched frictional melt produced during seismic slip) and mylonites (produced during the post- and interseismic viscous creep) provide a snapshot of the earthquake cycle at anomalously deep conditions in the crust. Such natural laboratories offer the opportunity to investigate the origin and the tectonic setting of lower crustal earthquakes.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;span&gt;The Nusfjord East shear zone network (Lofoten, northern Norway) represents an exhumed lower crustal earthquake source, where mutually overprinting mylonites and pseudotachylytes record the interplay between coseismic slip and viscous creep (Menegon et al., 2017; Campbell and Menegon, 2019). The network is well exposed over an area of 4 km&lt;sup&gt;2&lt;/sup&gt; and consists of three main intersecting sets of ductile shear zones ranging in width from 1 cm to 1 m, which commonly nucleate on former pseudotachylyte veins. Mutual crosscutting relationships indicate that the three sets were active at the same time. Amphibole-plagioclase geothermobarometry yields consistent P-T estimates in all three sets (700-750 &amp;#176;C, 0.7-0.8 GPa). The shear zones separate relatively undeformed blocks of anorthosite that contain pristine pseudotachylyte fault veins. These pseudotachylytes link adjacent or intersecting shear zones, and are interpreted as fossil seismogenic faults representing earthquake nucleation as a transient consequence of ongoing, localised aseismic creep along the shear zones (Campbell et al., under review).&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;span&gt;The coeval activity of the three shear zone sets is consistent with a local extensional setting, with a bulk vertical shortening and a horizontal NNW-SSE extension. This extension direction is subparallel to the convergence direction between Baltica and Laurentia during the Caledonian Orogeny, and with the dominant direction of nappe thrusting in the Scandinavian Caledonides. &lt;sup&gt;40&lt;/sup&gt;Ar&amp;#8208;&lt;sup&gt;39&lt;/sup&gt;Ar dating of localized upper amphibolite facies shear zones in the Nusfjord area with similar orientation to the Nusfjord East network yielded an age range of 433&amp;#8211;413 Ma (Fournier et al., 2014; Steltenpohl et al., 2003), which indicates an origin during the collisional (Scandian) stage of the Caledonian Orogeny.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;span&gt;We propose that the Nusfjord East brittle-viscous extensional shear zone network represents the rheological response of the lower crust to the bending of the lower plate during continental collision. (Micro)seismicity in the lower crust in collisional orogens is commonly localized in the lower plate and has extensional focal mechanisms. This has been tentatively correlated with slab rollback and bending of the lower plate (Singer et al., 2014). We interpret the Nusfjord East shear zone network as the geological record of this type of lower crustal seismicity.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;

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