Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula

2020 ◽  
Author(s):  
Marcos Marcén ◽  
Antonio Casas-Sainz ◽  
Teresa Román-Berdiel ◽  
Belén Oliva-Urcia ◽  
Ruth Soto ◽  
...  
Keyword(s):  
Shear Zones ◽  
Fault Zones ◽  
Magnetic Fabric ◽  
Fault Rock ◽  
Fault Rocks ◽  
Magnetic Lineation ◽  
Magnetic Fabrics ◽  
Transport Direction ◽  
Wide Range ◽  
Magnetic Lineations

<p>Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim  to interpret patterns of AMS in cataclasites.</p><p>Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).</p><p>The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.</p>

Magnetic fabric in carbonatic rocks from thrust shear zones

2021 ◽  
Author(s):  
Sara Satolli ◽  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Dorota Staneczek ◽  
Fernando Calamita ◽  
...  
Keyword(s):  
Cluster Analysis ◽  
Pure Shear ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Structural Deformation ◽  
Axis Orientation ◽  
Main Thrust ◽  
Magnetic Lineation ◽  
Magnetic Fabrics ◽  
And Cluster Analysis

<p><strong> </strong></p><p>The aim of this study is to investigate how structural deformation in shear zones is documented by the anisotropy of magnetic susceptibility (AMS). The study area is located in the Pliocene outer thrust of the Northern Apennines, which involved Cretaceous to Neogene calcareous and marly rocks. Here, brittle-ductile tectonites show different characteristics along two differently oriented thrust ramps: the NNE-SSW-trending oblique thrust ramp is characterized by the presence of S tectonites, while the NW-SE-trending frontal ramp is characterized by the presence of SC tectonites.</p><p>Samples for AMS fabric investigation were collected on shear zones from three sectors of the belt, at different distance from the main thrust to detect possible magnetic fabric variations. The three study area are characterized by different combinations of simple and pure shear, thus different degree of non-coaxiality, which has been quantified through the vorticity number W<sub>k</sub>.</p><p>Specimens were measured with an AGICO KLY-3 Kappabridge at the CIMaN-ALP Laboratory (Italy) on 15 different directions mode. Only measurements with all three F-statistics of the anisotropy tests higher than 5 were accepted as reliable. Moreover, outliers characterized by ± 2σ difference with respect to the mean value of AMS scalar parameters were excluded from further analysis. In order to distinguish groups of specimens affected by different sedimentary or tectonic processes, we identified clusters of AMS scalar parameters; when clusters were not defined by these parameters, we applied a combination of contouring and cluster analysis on each principal axis to identify different subfabrics.</p><p>The magnetic fabric revealed straightforward correlations with structural data and specific changes of AMS axis orientation depending upon the increasing of deformation (lower vorticity number) and proximity to the main thrust. Similar evolution was detected in different deformation regimes. Overall, the magnetic fabric is more sensitive to the simple shear deformation, as the magnetic lineation tends to parallelize mostly with the computed slip vector; however in pure-shear dominated regimes, the magnetic lineation becomes parallel to the transport direction when the deformation is really intense (sites at less than 15-30 cm from the thrust plane).</p><p>The applied combination of density diagrams and cluster analysis on AMS data successfully allowed discriminating subfabrics related to different events, and shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics.</p>

scholarly journals Magnetic properties of pseudotachylytes, Jämtland, central Sweden

2019 ◽  
Author(s):  
Hagen Bender ◽  
Bjarne S. G. Almqvist ◽  
Amanda Bergman ◽  
Uwe Ring
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Fault Zones ◽  
Small Sample ◽  
Magnetic Fabric ◽  
Normal Faults ◽  
Magnetic Fabrics ◽  
Nappe Complex ◽  
Fault Kinematics

Abstract. Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny

2020 ◽  
Author(s):  
Helena Sant Ovaia ◽  
Ana Gonçalves ◽  
Claudia Cruz ◽  
Fernando Noronha
Keyword(s):  
Stress Field ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Magnetic Fabric ◽  
Variscan Orogeny ◽  
Maximum Compression ◽  
Compression Direction ◽  
Magnetic Fabrics ◽  
Central Portugal ◽  
Magnetic Lineations

<p>This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D<sub>1</sub>, D<sub>2</sub> and D<sub>3</sub>, being the variscan magmatism events mainly related to D<sub>3</sub> phase. D<sub>3</sub> produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, σ1, NE-SW oriented. The post-D<sub>3</sub> brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D<sub>3</sub> phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D<sub>3</sub> two-mica granites, ca. 311 Ma; (2) late-D<sub>3</sub> monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D<sub>3</sub> monzogranites and biotite-rich granites, ca. 299 – 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D<sub>3</sub> granites show magnetic foliations and lineations consistent with the syn-D<sub>3</sub> variscan structures ca. N110°-120°E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60º), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D<sub>3</sub> granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60º) and the lineations have, generally, soft plunges. (iii) Post-D<sub>3</sub> granites have, in general, magnetic foliations and lineations associated with important regional post-D<sub>3</sub> brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60º), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.</p><p>Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).</p>

On the straight and narrow: extreme localization of brittle fault damage and displacement along the Glade Fault Zone, Eastern Fiordland, New Zealand

2020 ◽  
Author(s):  
Michael Ofman ◽  
Steven Smith
Keyword(s):  
New Zealand ◽  
Fault Zone ◽  
Damage Zone ◽  
Shear Zones ◽  
Strike Slip ◽  
Fault Rock ◽  
Fault Rocks ◽  
X Ray ◽  
Fracture Damage ◽  
Host Rocks

<p>The southern Glade Fault Zone is a crustal-scale, subvertical dextral strike-slip fault zone on the eastern margin of Fiordland, New Zealand. For a distance of c. 40 km between Lake Te Anau and the Hollyford Valley, the fault cuts plutonic host rocks and has an estimated total dextral separation of c. 6-8 km. We report previously unidentified mylonites, cataclasites, pseudotachylites and fault gouge subparallel to pervasive sets of planar cooling joints in the Hut Creek-Mistake Creek area plutonic suites. The outcropping assemblage of joints and fault rocks record thermal, seismic and rheological conditions in the southern Glade Fault. Here we integrate methods to characterise the fault rocks and fracture damage zone of the southern Glade Fault from Glade Pass to Mt Aragorn. We use (i) EDS (Energy Dispersive x-ray Spectroscopy), XRD (X-Ray Diffraction) and EBSD (Electron Backscatter Diffraction) analysis to describe the mineralogy, kinematics and microstructures of fault rocks and, (ii) drone orthophotography and traditional structural measurements to detail geometrical relationships between structural features. Field mapping of glacially polished outcrops identifies the zone of brittle fault-related damage (i.e. damage zone + fault rock sequence) is up to one order of magnitude narrower than documented along other strike-slip faults with similar displacements, suggesting that the Glade Fault Zone represents an “end-member” of extreme localization of brittle deformation and fault displacement. This is interpreted to result from linkage of pre-existing cooling joints (and mylonitic shear zones), which allowed the younger brittle fault zone to establish its length and planarity relatively efficiently compared to the case of fault nucleation and growth in more isotropic host rocks.</p>

scholarly journals Determination of the Youngest Active Domain in Major Fault Zones Using Medical X-ray Computed Tomography-derived Density Estimations

2021 ◽  
Author(s):  
akiyuki iwamori ◽  
Hideo Takagi ◽  
Nobutaka Asahi ◽  
Tatsuji Sugimori ◽  
Eiji Nakata ◽  
...  
Keyword(s):  
Fault Zone ◽  
Atomic Number ◽  
Density Ratio ◽  
Effective Atomic Number ◽  
Fault Zones ◽  
Ct Number ◽  
Fault Rock ◽  
Fault Rocks ◽  
X Ray Computed

Abstract Determination of the youngest active domains in fault zones that are not overlain by Quaternary sedimentary cover are critical for evaluating recent fault activity, determining the current local stress field, and mitigating the impacts of future earthquakes. Considering the exhumation of a fault zone, the youngest active domain in a fault zone is supposed to correspond to the activity at the minimum fault depth of a buried fault, such that the most vulnerable area, which possesses the lowest rock/protolith density ratio, is assumed to be indicative of this recent fault activity. However, it is difficult to measure the density of fault rocks and map the rock/protolith density ratio across a given fault zone. Here we utilize medical X-ray computed tomography (CT), a non-destructive technique for observing and analyzing materials, to investigate the fault characteristics of several fault zones and their surrounding regions in Japan, and attempt to determine the youngest active domain of a given fault zone based on its CT numbers, which are a function of the density and effective atomic number of the fault rock and protolith. We first investigate the density, void ratio, and effective atomic number of active and inactive fault rocks, and their respective protoliths. We then calculate the CT numbers after reducing the beam-hardening effects on the rock samples, and study the relationships among the CT number, density, and effective atomic number. We demonstrate that the density, effective atomic number, and CT number of the fault rock decrease as the youngest active zone is approached, such that the region with the lowest CT number and rock/protolith density ratio defines the youngest active domain of a given fault zone.

Permeability of carbonate fault rocks: a case study from Malta

2019 ◽  
Vol 26 (3) ◽  
pp. 418-433 ◽  
Author(s):  
Andy P. Cooke ◽  
Quentin J. Fisher ◽  
Emma A. H. Michie ◽  
Graham Yielding
Keyword(s):  
Fluid Flow ◽  
Host Rock ◽  
Damage Zone ◽  
Fault Zones ◽  
High Porosity ◽  
Rock Permeability ◽  
Fault Rock ◽  
Fault Rocks ◽  
Diagenetic Alteration ◽  
Hydraulic Behaviour

The inherent heterogeneity of carbonate rocks suggests that carbonate-hosted fault zones are also likely to be heterogeneous. Coupled with a lack of host–fault petrophysical relationships, this makes the hydraulic behaviour of carbonate-hosted fault zones difficult to predict. Here we investigate the link between host rock and fault rock porosity, permeability and texture, by presenting data from series of host rock, damage zone and fault rock samples from normally faulted, shallowly buried limestones from Malta. Core plug X-ray tomography indicates that texturally heterogeneous host rocks lead to greater variability in the porosity and permeability of fault rocks. Fault rocks derived from moderate- to high-porosity (>20%) formations experience permeability reductions of up to six orders of magnitude relative to the host; >30% of these fault rocks could act as baffles or barriers to fluid flow over production timescales. Fault rocks derived from lower-porosity (<20%) algal packstones have permeabilities that are lower than their hosts by up to three orders of magnitude, which is unlikely to impact fluid flow on production timescales. The variability of fault rock permeability is controlled by a number of factors, including the initial host rock texture and porosity, the magnitude of strain localization, and the extent of post-deformation diagenetic alteration. Fault displacement has no obvious control over fault rock permeability. The results enable better predictions of fault rock permeability in similar lithotypes and tectonic regimes. This may enable predictions of across-fault fluid flow potential when combined with data on fault zone architecture.

scholarly journals Carbonate clumped isotope thermometry of fault rocks and its possibilities: tectonic implications from calcites within Himalayan Frontal Fold-Thrust Belt

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Dyuti Prakash Sarkar ◽  
Jun-ichi Ando ◽  
Akihiro Kano ◽  
Hirokazu Kato ◽  
Gautam Ghosh ◽  
...  
Keyword(s):  
Fault Zone ◽  
Analytical Techniques ◽  
Fault Zones ◽  
Low Grade ◽  
Fault Rock ◽  
Fault Rocks ◽  
Main Boundary Thrust ◽  
Element Partitioning ◽  
Clumped Isotope ◽  
Northwest India

AbstractDisentangling the temperature and depth of formation of fault rocks is critical for understanding their rheology, exhumation, and the evolution of fault zones. Estimation of fault rock temperatures mostly relies on conventional geothermometers of metamorphic minerals and element partitioning analysis, which are largely inapplicable in shallow crustal fault rocks. Here, we demonstrate the applicability of the carbonate clumped isotope thermometer in low-grade carbonate-bearing fault rocks from the Himalayan frontal wedge (northwest India). Coalescing carbonate clumped isotope thermometry and calcite e-twin morphology allows us to constrain the temperature and depth of formation of the two main thrusts of the Himalayan frontal wedge, the Nahan thrust (170 ± 10 °C; 6–7 km depth), and the Main Boundary thrust (262 ± 30 °C; 10–11 km depth). The integration of the adopted analytical techniques can promote the application of calcite-based clumped isotope thermometry to the fault zone processes and refinement of shallow crustal fault zone models.

Deformation and magnetic fabric of the Capinha granite (Fundão, Central Portugal): ascent and emplacement mechanisms during the late-Variscan crustal thinning

2020 ◽  
Author(s):  
Ana Gonçalves ◽  
Helena Sant'Ovaia ◽  
Fernando Noronha
Keyword(s):  
Magnetic Susceptibility ◽  
Major Axis ◽  
Magnetic Fabric ◽  
Variscan Orogeny ◽  
Magnetic Lineation ◽  
Metasedimentary Rocks ◽  
Magnetic Fabrics ◽  
Crustal Thinning ◽  
Deformation Patterns ◽  
Magnetic Foliation

&lt;p&gt;The Capinha area is located in the Central Iberian zone and is characterized by several Variscan granites intruded in the Neoproterozoic&amp;#8211;Cambrian metasedimentary rocks. The main goal of the study is to identify the deformation patterns and provide crucial information to investigate the evolution of the magnetic fabrics in a post-Variscan granite emplaced during the crustal thinning, at the end of the Variscan orogeny. In order to achieve these purposes, fieldwork, petrography, microstructures and anisotropy of magnetic susceptibility (AMS) analysis were undertaken. The AMS was measured in 160 oriented cores, collected from 20 sampling sites homogeneously distributed, allowing the quantification of scalar (magnetic susceptibility, K; paramagnetic anisotropy, P&lt;sub&gt;para&lt;/sub&gt;; magnetic ellipsoid shape, T) and directional data (magnetic lineation, //K&lt;sub&gt;1&lt;/sub&gt;; magnetic foliation, perpendicular to K&lt;sub&gt;3&lt;/sub&gt;). The Capinha granite (CG), exposed over an area of about 7 km&lt;sup&gt;2&lt;/sup&gt;, is a small circular circumscribed outcrop in the NE-SW contact between the regional Belmonte&amp;#8211;Caria granite (301.1&amp;#177;2.2 Ma) and the metasedimentary sequences. The CG is cut by two main fracturing systems: N30&amp;#186;-40&amp;#186;E and N110&amp;#186;-120&amp;#186;E, both subvertical. The contact is sharp, intrusive and discordant with the general trending of the D&lt;sub&gt;1&lt;/sub&gt; and D&lt;sub&gt;3&lt;/sub&gt; Variscan structures registered in the metasedimentary rocks. The CG is homogeneous in the whole area and consists of a fine- to medium-grained, muscovite-biotite leucogranite. The CG exhibit a paramagnetic behaviour with a K mean of 73 &amp;#181;SI, belonging to the ilmenite-type granites. At several scales, the CG does not show any magmatic flow or ductile deformation patterns displaying P&lt;sub&gt;para&lt;/sub&gt; of about 1.6%, which corresponds to dominant magmatic to submagmatic microstructures. The P&lt;sub&gt;para&lt;/sub&gt; highest values are concentrated in the NE border associated to prolate ellipsoids (linear fabric). Based on the interpretation of the magnetic fabric, is possible to observe that the orientation of the magnetic foliation is variable ranging from NNW-SSE to NNE-SSW. Generally, the magnetic foliations are sub-horizontal, being the vertical dips observed in the NE border, near the intersection of the N100&amp;#186;-120&amp;#186;E and the N30&amp;#186;-40&amp;#186;E fractures. The arrangement of the magnetic foliations follow concentric trajectories, with the symmetry axe parallel to the major axis of the outcrop (roughly NNE-SSW). The magnetic lineations are mainly sub-horizontal NNE-SSW parallel to the granite major axis; although, in the SW border the lineations tend to be parallelized to the contact. The magnetic lineation arrangement develops linear trajectories converging to the NE zone, where the dip is strong. The common gently magnetic fabric suggests the roof of the CG intrusion. During the late stages of the Variscan orogeny (D&lt;sub&gt;3&lt;/sub&gt;, 321-300 Ma), ductile extensional detachments promoted the thinning of a previously thickened crust, providing the opening of pre-existing structures and the production of new ones. These structures act as conduits for a passive magma ascending and emplacement at shallow levels. Therefore, it is suggested that the CG magma ascent and emplaced in the intersection of pre-existing fractures, located in the NE zone, and flowed to the SW, developing a small asymmetric laccolith, poorly eroded, with a tongue-shaped body.&lt;/p&gt;

scholarly journals Fluid-driven Cyclic Reorganisation in Shallow Basaltic Fault Zones

2021 ◽  
Author(s):  
Bob Bamberg ◽  
Richard Walker ◽  
Marc Reichow ◽  
Audrey Ougier-Simonin
Keyword(s):  
Host Rock ◽  
Shear Bands ◽  
Shear Zones ◽  
Fault Zones ◽  
Fault Rock ◽  
Rock Composition ◽  
Multi Scale ◽  
Riedel Shear ◽  
And Migration ◽  
Rock Microstructure

Faults represent a critical heterogeneity in basaltic sequences, yet their architectural and hydromechanical evolution is poorly constrained. We present a detailed multi-scale characterisation of passively exhumed fault zones from the layered basalts of the Faroe Islands, which reveals cyclic stages of fault evolution. Outcrop-scale structures and fault rock distribution within the fault zones were mapped in the field and in 3D virtual outcrop models, with detailed characterisation of fault rock microstructure obtained from optical and SE-microscopy. The fault zones record localisation from decametre-wide Riedel shear zones into metre-wide fault cores, containing multiple cataclastic shear bands and low strain lenses organised around a central principal slip zone (PSZ). Shear bands and the PSZ consist of (ultra-) cataclasites with a zeolite-smectite assemblage replacing the original plagioclase-pyroxene host rock composition. Low-strain lenses are hydrothermal breccias of weakly altered host rock, or reworked fault rocks. PSZ-proximal zones show significant late-stage dilatation in the form of hydrothermal breccias or tabular veins with up to decimetre apertures. We interpret these structures as evolving from alternating shear-compaction and dilation through hydrofracture. The fault core preserves PSZ reworking, evidencing repeated shear zone locking and migration. The alternating deformation styles of shear compaction and dilatation suggest episodic changes in deformation mechanisms driven by transient overpressure and release. The fault zone mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing.

Effect of kinematics of orogenic wedge on kinematic evolutionary paths and deformation profiles of major shear zones: An example from the eastern Himalaya 

2021 ◽  
Author(s):  
Pritam Ghosh ◽  
Kathakali Bhattacharyya
Keyword(s):  
Shear Zone ◽  
Strain Softening ◽  
Leading Edge ◽  
Shear Zones ◽  
Deformation History ◽  
Progressive Deformation ◽  
Trailing Edge ◽  
Orogenic Wedge ◽  
Transport Direction ◽  
Evolutionary Paths

&lt;p&gt;We examine how the deformation profile and kinematic evolutionary paths of two major shear zones with prolonged deformation history and large translations differ with varying structural positions along its transport direction in an orogenic wedge. We conduct this analysis on multiple exposures of the internal thrusts from the Sikkim Himalayan fold thrust belt, the Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex (LHD), and the overlying Main Central thrust (MCT). These two thrusts are regionally folded due to growth of the LHD and are exposed at different structural positions. The hinterlandmost exposures of the MCT and PT zones lie in the trailing parts of the duplex, while the foreland-most exposures of the same studied shear zones lie in the leading part of the duplex, and thus have recorded a greater connectivity with the duplex. The thicknesses of the shear zones progressively decrease toward the leading edge indicating variation in deformation conditions. Thickness-displacement plot reveals strain-softening from all the five studied MCT and the PT mylonite zones. However, the strain-softening mechanisms varied along its transport direction with the hinterland exposures recording dominantly dislocation-creep, while dissolution-creep and reaction-softening are dominant in the forelandmost exposures. Based on overburden estimation, the loss of overburden on the MCT and the PT zones is more in the leading edge (~26km and ~15km, respectively) than in the trailing edge (~10km and ~17km, respectively), during progressive deformation. Based on recalibrated recrystallized quartz grain thermometer (Law, 2014), the estimated deformation temperatures in the trailing edge are higher (~450-650&amp;#176;C) than in the leading edge (350-550&amp;#176;C) of the shear zones. This variation in the deformation conditions is also reflected in the shallow-crustal deformation structures with higher fracture intensity and lower spacing in the leading edge exposures of the shear zones as compared to the trailing edge exposures.&lt;/p&gt;&lt;p&gt;The proportion of mylonitic domains and micaceous minerals within the exposed shear zones increase and grain-size of the constituent minerals decreases progressively along the transport direction. This is also consistent with progressive increase in mean R&lt;sub&gt;s&lt;/sub&gt;-values toward leading edge exposures of the same shear zones. Additionally, the &amp;#945;-value (stretch ratio) gradually increases toward the foreland-most exposures along with increasing angular shear strain. Vorticity estimates from multiple incremental strain markers indicate that the MCT and PT zones generally record a decelerating strain path. Therefore, the results from this study are counterintuitive to the general observation of a direct relationship between higher Rs-value and higher pure-shear component. We explain this observation in the context of the larger kinematics of the orogen, where the leading edge exposures have passed through the duplex structure, recording the greatest connectivity and most complete deformation history, resulting in the weakest shear zone that is also reflected in the deformation profiles and strain attributes. This study demonstrates that the same shear zone records varying deformation profile, strain and kinematic evolutionary paths due to varying deformation conditions and varying connectivity to the underlying footwall structures during progressive deformation of an orogenic wedge.&lt;/p&gt;

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