scholarly journals CONTRASTING GEOMETRY BETWEEN ALPINE AND LATE- TO POSTALPINE TECTONIC STRUCTURES IN ANAFI ISLAND (CYCLADES)

2004 ◽  
Vol 36 (4) ◽  
pp. 1688 ◽  
Author(s):  
Konstantinos I. Soukis ◽  
Demetrios J. Papanikolaou
Keyword(s):  
Normal Faults ◽  
Thrust Faults ◽  
High Angle ◽  
Backarc Basin ◽  
Tectonic Structures ◽  
Extensional Deformation ◽  
Upper Miocene ◽  
Deformation Phase ◽  
Shear Sense ◽  
Shear Sense Indicators

A significant change is observed in the geometry of the major faults in Anafi Island from Ν to NE dip in the alpine structures to S to SW dip in the late to post-alpine. Several thrust faults dipping to the NE preserved at the central-eastern part of the island form a nappe pile of Cretaceous HT/LP metamorphic units emplaced over a parautochthonous nonmetamorphic flysch of partly Eocene age without development of metamorphic structures (deformation phase Di). The development of detachment normal faults dipping to the SW (deformation phase D2) deformed the previous thrusts together with the post alpine continental sedimentary sequence of Miocene age, occurring in the northern and western part of Anafi. Asymmetric to the SW folds are observed at the lower part of the Upper Miocene sediments as well as numerous shear sense indicators along the undulating surface of the detachment above the underlying alpine units. The deformation weakens towards the upper part of the Upper Miocene sediments. An intensely sheared molassic type formation of probable ?Oligocene-Early Miocene age was distinguished between the alpine units and the detached Upper Miocene sediments. High angle normal faults dipping to the SW deform all previous structures (D3). The extensional deformation phases D2 and D3 are related to the opening of the Cretan backarc basin during Tortonian - Early Pliocene.

From the hydrogeological and geochemical conceptualisation to the groundwater management: the Gioia Tauro Plain (Southern Italy)

2021 ◽  
Author(s):  
Giuseppe Cianflone ◽  
Giovanni Vespasiano ◽  
Rosanna De Rosa ◽  
Carmine Apollaro ◽  
Rocco Dominici ◽  
...  
Keyword(s):  
Fault Zone ◽  
Southern Italy ◽  
Groundwater Resources ◽  
Shallow Aquifer ◽  
Normal Faults ◽  
Thickness Variation ◽  
High Angle ◽  
Deep Aquifer ◽  
Upper Miocene ◽  
Geophysical Surveys

<p>The Gioia Tauro plain (GTP) is an industrialized and agricultural coastal area of about 500 km<sup>2</sup> in the Tyrrhenian side of Calabria. Its harbour is one of the most important container traffic hubs in the Mediterranean basin. The GTP groundwater resources are constantly at risk of depletion and quality degradation due to anthropic activities.</p><p>GTP is a half-graben bounded by two massifs. The boundaries are marked by three main fault systems: the Nicotera-Gioiosa fault zone, NW-SE striking and right lateral kinematics along the north boundary; the NNE-SSW Cittanova Fault, a high-angle normal and active fault along the eastern border; the Palmi-Locri fault zone with NW-SE trend and a mainly strike-slip kinematics along the south boundary. The GTP sedimentary infill is made by an upper Miocene siliciclastic and carbonate succession overlays by Pliocene marly-limestone rhytmites and Piacenzian-Calabrian sandstones and calcarenites with interbedded 20m thick volcaniclastic deposits. Upward, the sedimentary infill continues with alluvial (in eastern and middle sector) and coastal (in the western sector) deposits.</p><p>Six geochemical facies of groundwater were distinguished, with different salinities and temperatures (Italiano et al., 2010). The majority of samples is of cold shallow groundwater and shows Ca-HCO<sub>3</sub>, Ca(Mg-Na)-HCO<sub>3</sub>(Cl-SO<sub>4</sub>) and Na-HCO<sub>3</sub> composition and overall low salinities (TDS <1g / L). Only few samples, with Na-SO<sub>4</sub> and Na-Cl composition, show high salinity (TDS <3.5g / L) and temperature (above 20 ° C). These latter occur in the northern portion of the plain, near the intersection of the Palmi-Gioia Tauro and Nicotera-Gioiosa faults systems, and in the southern sector, near Palmi town.</p><p>It was created a geodatabase using data of hundreds of boreholes, geotechnical and geophysical investigations. Furthermore, it is carrying out a geological and geophysical survey along the plain boundaries using passive seismic technique to infer the deep of discontinuities among the main geological units described above. The acquired data allowed to identify: i) the shallow aquifer, made by Pleistocene-Holocene deposits characterized by complex lateral variations; ii) at the bottom, the aquitard, represented by Pliocene marls; iii) the deep aquifer, consisting of the upper Miocene succession. The highest thickness of shallow aquifer (more than 200 m) is observed in the middle GTP sector. The thickness variation is strictly related to the NE-SW high angle normal faults which cross the GTP. The ongoing geological, geochemical, and geophysical surveys will allow: i) to identify the geometry of the hydrogeological units; ii) to define the hydrogeological features of the groundwater systems useful for modelling purposes, and iii) to improve the knowledge of water rock interactions processes (e.g., relations between deep and shallow waters, anthropogenic effects, seawater intrusion) for management purposes.</p><p>Italiano, F. et al. 2010. Geochemistry of fluids discharged over the seismic area of the Southern Apennines (Calabria region, Southern Italy): Implications for fluid-fault relationships. Appl. Geochem. 25, 540–554.</p>

scholarly journals THE NAPPE STRUCTURE OF THE TECTONIC WINDOW OF DOLIANA (CENTRAL PELOPONNESUS, GREECE)

2004 ◽  
Vol 36 (4) ◽  
pp. 1662 ◽  
Author(s):  
S. Lekkas ◽  
E. Skourtsos
Keyword(s):  
The Other ◽  
Metamorphic Rocks ◽  
Normal Faults ◽  
High Angle ◽  
Lower Miocene ◽  
The Core ◽  
Lower Oligocene ◽  
Deformation Phase ◽  
Tectonic Window ◽  
Geological Formations

In the tectonic window of Doliana (central Peloponnesus, Greece) a variety of geological formations occurs deriving from different sedimentation palaeoenvironments and characterized by different tectonometamorphic evolution. These rocks initially formed thrust nappes of great extension and thickness which stacked one over the other during the Lower Oligocene-Lower Miocene. The early compressing structures that were formed by the emplacement of the nappes were almost totally overprinted by the later-orogenic extension that affected the nappe-column in the Upper Miocene-Lower Pliocene. Its main tectonic characteristic was the formation of low-angle normal faults, which constitute the tectonic contacts of the nappes. These faults led to the exhumation of the formerly deeply buried metamorphic rocks that are forming the core of the tectonic window. Apart from these structures during the evolution of this deformation phase an intense thinning of the units of the upper plate took place, causing the upper units coming closer or next to the lower ones. After the Lower Pliocene a second extensional phase affected the already thinned nappe-column with the formation of high-angle normal faults.

Dike swarms on Seward Peninsula, Alaska, and their implications for the kinematics of Cretaceous extension in the Bering Strait region

2003 ◽  
Vol 40 (6) ◽  
pp. 865-886 ◽  
Author(s):  
Jeffrey M Amato ◽  
Elizabeth L Miller ◽  
James E Wright ◽  
William C McIntosh
Keyword(s):  
Late Cretaceous ◽  
Tectonic Setting ◽  
Ductile Deformation ◽  
Geochemical Data ◽  
Normal Faults ◽  
Bering Strait ◽  
Shear Sense ◽  
Seward Peninsula ◽  
Dike Swarms ◽  
Shear Sense Indicators

Late Cretaceous dike swarms on Seward Peninsula, northwestern Alaska, represent the youngest local manifestation of a ~115–75 Ma magmatic event in the Bering Strait region. Magmatism accompanied and followed high-grade metamorphism and ductile deformation. A Late Cretaceous extensional tectonic setting for the region is suggested by the thickness and seismic-reflection characteristics of the crust, regional basin development, formation of high-strain tectonites with subhorizontal foliations, bimodal magmatism, and dike swarms. The orientation of the dike swarms is used to address the kinematics of extension. A diabase dike swarm in the Kigluaik Mountains consists of dikes that strike northeast (040°) and dip steeply. Phenocrysts include plagioclase, clinopyroxene, orthopyroxene, and hornblende. Geochemical data indicate that SiO2 ranges from 48% to 56%, and K2O from 1.2% to 4.0%. The dikes are geochemically similar to the mafic to intermediate root of the 90 Ma Kigluaik pluton. Sr- and Nd-isotope data show that initial 87Sr/86Sr ranges from 0.7070 to 0.7077 and initial εNd ranges from –0.85 to –2.90. Field relations and 40Ar/39Ar geochronology bracket the dike ages between 90 and 84 Ma. Diabase dikes in the York Mountains are associated with normal faults that strike east–west to east-northeast. Dikes in the Bendeleben Mountains are both mafic and felsic, but their orientations are unknown. Alkalic dikes in the Darby Mountains strike 030°–050°, similar to those in the Kigluaik Mountains. Regional relationships including the orientation of dikes, normal faults, mineral stretching lineations, and other shear-sense indicators suggest that between 110 and 90 Ma extension on Seward Peninsula was generally oriented north–south to north-northwest–south-southeast.

A reinterpretation of the Snow Lake gold camp, Trans-Hudson Orogen, Canada: The use of cleavages as markers to correlate structures across deformed terranes

2020 ◽  
Author(s):  
Kate Elizabeth Rubingh ◽  
Bruno Lafrance ◽  
Harold L. Gibson
Keyword(s):  
Boundary Conditions ◽  
Time Lag ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Thrust Faults ◽  
Volcanic Stratigraphy ◽  
Shear Sense ◽  
Flin Flon ◽  
Shear Sense Indicators ◽  
Superior Craton

The Snow Lake gold camp is located within amphibolite facies volcanic rocks of the ca. 1.88 – 1.87 Ga Flin Flon-Glennie Complex (FFGC) in the Trans-Hudson Orogen, Manitoba. During thrusting and collision with the Archean Sask craton, volcanic rocks were interleaved with turbidites of the ca. 1.855 - 1.84 Ga Burntwood Group and sandstone and conglomerate of the ca. 1.845 - 1.835 Ga Missi Group. The main cleavage in the turbidites was previously attributed to thrusting and used as a marker for correlating structures across the camp. A re-examination of this cleavage suggests that it overprints the thrust faults and formed during later collision between the FFGC and the Archean Superior craton. This has important implications as it further suggests that (1) previously unrecognized, early brittle thrust faults repeat volcanic stratigraphy and may have created the boundary conditions that enabled the formation of ductile thrust faults, fold nappes, and mega sheath folds; (2) shear sense indicators along ductile thrust faults formed during their reactivation as sinistral shear zones rather than during thrusting; and (3) peak metamorphic conditions were caused by thrusting and stacking during collision with the Sask craton but were attained later during collision with the Superior craton due to the time lag between orogenesis and the re-equilibration of regional isotherms. Results from this study may be applicable to other complexly deformed terranes where the dominant regional cleavage differs in expression in mixed volcanic and sedimentary successions and has been used as a marker for correlating structures.

scholarly journals Geology of the orogenic Cheminis gold deposit along the Larder Lake – Cadillac deformation zone, Ontario

2015 ◽  
Vol 52 (12) ◽  
pp. 1093-1108 ◽  
Author(s):  
Bruno Lafrance
Keyword(s):  
Deformation Zone ◽  
Volcanic Rocks ◽  
Tholeiitic Basalt ◽  
Hydrothermal Fluids ◽  
South Side ◽  
Shear Sense ◽  
Abitibi Subprovince ◽  
Deformation Features ◽  
Shear Sense Indicators ◽  
Host Rocks

The Larder Lake – Cadillac deformation zone (LLCDZ) is one of two major, auriferous, deformation zones in the southern Abitibi subprovince of the Archean Superior Province. It hosts the Cheminis and the giant Kerr Addison – Chesterville deposits within a strongly deformed band of Fe-rich tholeiitic basalt and komatiite of the Larder Lake Group (ca. 2705 Ma). The latter is bounded on both sides by younger, less deformed, Timiskaming turbidites (2674–2670 Ma). The earliest deformation features are F1 folds affecting the Timiskaming rocks, which formed either during D1 extensional faulting or during early D2 north–south shortening related to the opening and closure, respectively, of the Timiskaming basin. Continued shortening during D2 imbricated the older volcanic rocks and turbidites and produced regional F2 folds with an axial planar S2 cleavage. D2 deformation was partitioned into the weaker band of volcanic rocks, producing the strong S2 foliation, L2 stretching lineation, and south-side-up shear sense indicators, which characterize the LLCDZ. Gold is present in quartz–carbonate veins in deformed fuchsitic komatiites (carbonate ore) and turbiditic sandstone (sandstone-hosted ore), and in association with disseminated pyrite in altered Fe-rich tholeiitic basalts (flow ore). All host rocks underwent strong mass gains in CO2, S, K2O, Ba, As, and W, during sericitization, carbonatization, and sulphidation of the host rocks, suggesting that they interacted with the same hydrothermal fluids. Textural relationships between alteration minerals and S2 cleavage indicate that mineralization is syn-cleavage. Thus, gold was deposited as hydrothermal fluids migrated upward along the LLCDZ during contractional, D2 south-side-up shearing. The gold zones were subsequently modified during D3 reactivation of the LLCDZ as a dextral transcurrent fault zone.

The pattern of the tectonic joints and the development of the Vlychada karst show cave in Diros, Peloponnesus, Greece

2021 ◽  
Author(s):  
Christos Filis ◽  
Emmanuel Skourtsos ◽  
Nikolaos Karalemas ◽  
Vasilios Giannopoulos ◽  
Ioannis Giannopoulos ◽  
...  
Keyword(s):  
Characteristic Feature ◽  
Sea Level ◽  
Carbonate Rocks ◽  
Meteoric Water ◽  
Weather Conditions ◽  
High Angle ◽  
Tectonic Structures ◽  
Joint System ◽  
Factors Influencing ◽  
Show Cave

<p>The most characteristic feature of carbonate rocks is that they are prone to dissolution due to the meteoric water circulation which is enriched in CO<sub>2</sub>. One of the factors influencing this phenomenon is the existence of discontinuities within the mass of carbonate rocks. The Diros Vlychada show cave, on the peninsula of Mani in Peloponnese, Greece, has developed in marbles that belong to the Plattenkalk geotectonic unit. Most of the cave is flooded with water and its level changes depending on the external weather conditions and variations in sea level. The deformation of the marbles is represented by tectonic structures formed during the Lower Miocene metamorphism and their subsequent exhumation. The final uplift stage took place during the Pliocene-Quaternary and is still active. Five joints systems were distinguished:</p><p>A NW-SE joint system which is subdivided into a subsystem with low-angle dips, mainly towards to the NW, related to the main foliation of the marbles and a second subsystem characterized by stretching joints of the same strike (elongated joints), which have high-angle dips, also towards the NW. The latter system intersects the former but is confined between marble bedding and does not extend to more than three beds (the bedding is defined by the first subsystem).</p><p>A NW-SE striking joint system characterized by stretching joints with high-angle dips, which intersects diagonally the two previous. This system extends between more than three marble beds.</p><p>Two systems show E-W and N-S strike with the first one much better expressed. Those joints have developed diagonally to the previous ones. These are mainly shear joints that intersect the first system and are propagated within many marble beds.</p><p>The chambers of the cave have been developed along NW-SE and E-W directions. The first one is identified with the joint system that has been developed transversely to the strike of the marble foliation and the second in parallel with the main system of the shear joints. It is interesting that the bays forming the coastline of the Mani peninsula, have developed in an E-W direction, which coincides with both one of the growth directions of the cave and one of the joints systems, which correspond to shear joints developed during the folding of the marbles. Stalactites and stalagmites grow in a NE-SW direction that is identical to the elongated joints which form the system that is parallel to the foliation strike. Groundwater flow along these branches may be slower as these branches appear to be restricted between marble bedding.</p>

scholarly journals Development of fault and vein networks in a carbonate sequence near Hayl al-Shaz, Oman Mountains

GeoArabia ◽  
2013 ◽  
Vol 18 (2) ◽  
pp. 99-136
Author(s):  
Simon Virgo ◽  
Max Arndt ◽  
Zoé Sobisch ◽  
Janos L. Urai
Keyword(s):  
United Arab Emirates ◽  
Coarse Grained ◽  
Normal Faults ◽  
Structural Elements ◽  
High Angle ◽  
Oman Mountains ◽  
Bedding Planes ◽  
Fracture Systems ◽  
Calcite Veins ◽  
Al Jabal Al Akhdar

ABSTRACT We present a high-resolution structural study on the dip slope of the southern flank of Jabal Shams in the central Oman Mountains. The objectives of the study were: (1) to test existing satellite-based interpretations of structural elements in the area; (2) prepare an accurate geological map; and (3) collect an extensive structural dataset of fault and bedding planes, fault throws, veins and joints. These data are compared with existing models of tectonic evolution in the Oman Mountains and the subsurface, and used to assess the applicability of these structures as analogs for fault and fracture systems in subsurface carbonate reservoirs in Oman. The complete exposure of clean rock incised by deep wadis allowed detailed mapping of the complex fault, vein and joint system hosted by Member 3 of the Cretaceous Kahmah Group. The member was divided into eight units for mapping purposes, in about 100 m of vertical stratigraphy. The map was almost exclusively based on direct field observations. It includes measurement of fault throw in many locations and the construction of profiles, which are accurate to within a few meters. Ground-truthing of existing satellite-based interpretations of structural elements showed that faults can be mapped with high confidence using remote-sensing data. The faults range into the subseismic scale with throws as little as a few decimeters. However, the existing interpretation of lineaments as cemented fractures was shown to be incorrect: the majority of these are open fractures formed along reactivated veins. The most prominent structure in the study area is a conjugate set of ESE-striking faults with throws resolvable from several centimeters to hundreds of meters. These faults contain bundles of coarse-grained calcite veins, which may be brecciated during reactivation. We interpret these faults to be a conjugate normal- to oblique fault set, which was rotated together with bedding during the folding of the Al Jabal al-Akhdar anticline. There are many generations of calcite veins with minor offset and at high-angle-to-bedding, sometimes in en-echelon sets. Analysis of clear overprinting relationships between veins at high-angle-to-bedding is consistent with the interpretations of Holland et al. (2009a); however we interpret the anticlockwise rotation of vein strike orientation to start before and end after the normal faulting. The normal faults post-date the bedding-parallel shear veins in the study area. Thus these faults formed after the emplacement of the Semail and Hawasina Nappes. They were previously interpreted to be of the same age as the regional normal- to oblique-slip faults in the subsurface of northern Oman and the United Arab Emirates, which evolved during the early deposition of the Campanian Fiqa Formation as proposed by Filbrandt et al. (2006). We interpret them also to be coeval with the Phase I extension of Fournier et al. (2006). The reactivation of these faults and the evolution of new veins was followed by folding of the Al Jabal al-Akhdar anticline and final uplift and jointing by reactivation of pre-existing microveins. Thus the faults in the study area are of comparable kinematics and age as those in the subsurface. However they formed at much greater depth and fluid pressures, so that direct use of these structures as analogs for fault and fracture systems in subsurface reservoirs in Oman should be undertaken with care.

Fluid flow and faulting history of the Iano tectonic window (Southern Tuscany, Italy).

2021 ◽  
Author(s):  
Paolo Fulignati ◽  
Martina Zucchi ◽  
Andrea Brogi ◽  
Enrico Capezzuoli ◽  
Domenico Liotta ◽  
...  
Keyword(s):  
Fluid Inclusions ◽  
Hydrothermal Fluids ◽  
Normal Faults ◽  
High Angle ◽  
Quartz Veins ◽  
History Of ◽  
Tectonic Window ◽  
Liquid Vapor

<p>In the Iano area (Southern Tuscany) a small tectonic window of Tuscan metamorphic units is observed. This belongs to the northernmost part of the so-called Mid-Tuscan ridge and, during Pliocene, formed a submarine high, now defining the easternmost shoulder of the Volterra Pliocene basin. The area gives the opportunity to investigate the complete cycle of negative inversion from crustal thickening to crustal thinning, which characterizes Southern Tuscany. Our new data focus on the western margin of the Iano ridge, and in particular on a system of high angle normal faults that represents the youngest structures of the investigated area. These structures, deformed low angle regional detachments locally juxtaposing the uppermost units of contractional nappe stack (the ophiolite-bearing Ligurian units), with the Tuscan metamorphic units, with an almost complete excision of at least 3.5 Km thick Mesozoic to Tertiary Tuscan nappe succession. The high angle normal faults show variable Plio-Quaternary vertical displacements from few meters to about 500 meters, and acted as pathways for the upwelling of hydrothermal fluids, as revealed by Pleistocene travertine deposits, hydrothermal alteration and occurrence of different generations of fluid inclusions in hydrothermal veins associated with these fault systems. Fluid inclusions were studied in quartz veins hosted in the Verrucano metasediments forming the top of the Tuscan metamorphic unit, as well as in some carbonate lithotypes (Cretaceous to Tertiary in age) of the overlying Tuscan Nappe. Two different kinds of fluid inclusions were documented. The Type 1 are multiphase (liquid + vapor + 1 daughter mineral) liquid-rich fluid inclusions whereas the Type 2 are two-phase (liquid + vapor) liquid-rich fluid inclusions. Type 1 fluid inclusions are primary in origin and were found only in quartz veins present in Verrucano metarudites, whereas Type 2 fluid inclusions occur in quartz veins present in both Verrucano phyllites and quartzites and in the carbonate units of the Tuscan Nappe. These are secondary and can be furthermore distinguished in two sub-populations (Type 2a and Type 2b) on the basis of petrographic observation and microthermometric data. Fluid inclusion investigation evidenced an evolution of the hydrothermal fluids from relatively high-T (~265°C) and hypersaline (35 wt.% NaCl<sub>equiv.</sub>) fluids trapped at about 100 MPa, to lower temperature (~195°C) and salinity (~9.5 wt.% NaCl<sub>equiv.</sub>) fluids, having circulated in the high-angle fault system. Based on the new data and a revision of the local tectonic setting a fluid-rock interaction history has been reconstructed with new hints and constraints for the Plio-Quaternary extensional history of the Volterra basin.</p>

Structural and microtectonic analyses of Barpak of Gorkha district, west-central Nepal

2020 ◽  
Vol 60 ◽  
pp. 163-179
Author(s):  
Sameer Poudel ◽  
Lok Mani Oli ◽  
Lalu P. Paudel
Keyword(s):  
Dynamic Recrystallization ◽  
Geological Mapping ◽  
Dolomitic Marble ◽  
Rock Types ◽  
Central Nepal ◽  
Phase Deformation ◽  
West Central ◽  
Shear Sense ◽  
Shear Sense Indicators ◽  
Cylindrical Folds

Geological mapping was carried out in the Barpak-Bhachchek area of the Daraudi River valley, Gorkha district, West-Central Nepal for structural analysis. The area comprises rocks of the Higher Himalayan Crystalline and the Lesser Himalayan Sequence.  Pelitic and psammitic schist, quartzite, calc-quartzite, dolomitic marble, graphitic schist, gneiss are the main rock types within the Lesser Himalayan Sequence,  whereas banded gneiss and quartzite form a significant portion of the Higher Himalayan Crystalline in the study area. The area is affected by poly-phase deformation. Lesser Himalayan Sequence has suffered five deformational phases (DL1-DL2, D3-D5) whereas the Higher Himalayan Crystalline has suffered four deformational events (DH1, D3-D5). The Lesser Himalayan Sequence lying to the northern limb of the Gorkha-Kuncha Anticlinorium is contort into doubly plunging to dome-and-basin-like en echelon type of non-cylindrical folds as Baluwa Dome and Pokharatar Basin (DL2 and D4). The direction of shearing as indicated by shear sense indicators (C' Shear band and Mica fish) is top-to-south coinciding with regional sense of shear related to the MCT propagation. The dynamic recrystallization direction, obtained from rock dominant with phyllosilicate minerals is top-to-south and coincides with mineral lineation and indicate the mineral lineation is contemporary with dynamic recrystallization during the MCT propagation.

Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones.

2021 ◽  
Author(s):  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Andrea Festa ◽  
Francesca Remitti
Keyword(s):  
Cluster Analysis ◽  
Hanging Wall ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Plate Boundaries ◽  
Thrust Faults ◽  
Localized Deformation ◽  
Main Thrust ◽  
Shear Sense ◽  
And Cluster Analysis

<p>Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.</p><p>As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.</p><p>Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.</p><p>We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.</p><p>Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).</p><p>Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.</p><p>Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).</p><p>An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.</p><p>Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.</p><p>In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.</p>

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