C-axis Fabrics Research Articles

Dynamically recrystallised quartz c-axis fabrics in greenschist facies quartzites, Singhbhum shear zone and its footwall, eastern India—influence of high fluid activity

Oriented specimens of quartz tectonites from the Singhbhum shear zone (SSZ), an important tectonic element in the Precambrian eastern Indian shield, and its footwall have been analysed for c-axis fabrics of dynamically recrystallised quartz grains. In individual specimens the fabric pattern is either an asymmetric type I cross-girdle or an asymmetric, kinked single-girdle, irrespective of whether the fabric is from the SSZ or its footwall. The asymmetry of the fabric confirms a southward thrust movement of the northern block on a northerly dipping shear zone. The measured fabrics are characterised by a concentration near the Y-axis of the finite strain ellipsoid with an equally or subordinately developed concentration near the Z-axis. Although some earlier works suggest the Y-concentration of the quartz c-axis fabric as due to the influence of higher temperature (obtained in amphibolite facies), the occurrence of a similar concentration in the quartz c-axis fabric from quartz tectonites of the SSZ and its footwall, interpreted to have formed under greenschist facies condition, is explained by formation of the fabric under high fluid activity during deformation.

Application of General Shear Theory to the Study of Formation Mechanism of the Metamorphic Core Complex: A Case Study of Xiaoqinling in Central China

Abstract: The kinematic vorticity number and strain of the mylonitic zone related to the detachment fault increase from ESE to WNW along the moving direction of the upper plate of the Xiaoqinling metamorphic core complex (XMCC) and the geometry of quartz c‐axis fabrics changes progressively from crossed girdles to single girdles in the same direction. Therefore, pure shear is dominant in the ESE part of the XMCC while simple shear becomes increasingly important towards WNW. However, the shear type does not change with the strain across the shear zone, thus the variation of shear type is of significance in indicating the formation mechanism. The granitic plutons within the XMCC came from the deep source and their emplacement was an active and forceful upwelling prior to the detachment faulting. The PTt path demonstrates that magmatism is an important cause for the formation of the XMCC. The formation mechanism of the XMCC is supposed to be active plutonism and passive detachment. Crustal thickening and magmatic doming caused necking extension with pure shear, and magmatic heating and doming resulted in detachment extension with simple shear and formed the XMCC.

Experimental work on the effect of pre-existing anisotropy on fabric development in glaciers

Abstract Localization of deformation in ice is known to be important at all scales in deforming glaciers. However, relatively little is known of the significance of shear localization and the influence of fabric development in anisotropic ice at microscopic scales (<mm-cm). In this experimental study, the effect of initial c -axis preferred orientation and the inclination of the primary layering in anisotropic ice masses, during both plane strain-compression and combined simple shear-compression, have been examined. A series of creep tests in the temperature range of −5 to −1°C over a range of shortening strains varying from 10 to 40% and compressive stresses ranging from 0 to 0.7 MPa have been undertaken. Significant variations in the strain rate and microstructural development have been observed in the plane strain-compression experiments that reflects the varying orientations of the anisotropy and its relationship to easy-glide directions in the ice mass. In the unconfined combined compression and shear experiments minimum shear stress rates vary between the variously oriented anisotropic ice masses and deviate from the normal power flow law for isotropic ice. Where annealing occurs, such ice masses preserve the pre-existing c -axis fabric and hence may reflect a contribution from both the recrystallized and inherited ice components.

The kinematic and metamorphic history of the Sgurr Beag Thrust, Ross-shire, NW Scotland

This paper presents a field and microstructural study of a Caledonian amphibolite-facies shear zone within the Moine rocks of Northern Scotland. The shear zone, the Sgurr Beag thrust, is an important tectonic break within the Moine nappe of Ross-shire and is the structurally highest and oldest of the foreland-propagating thrust system the youngest and lowest of which marks the Caledonian front in Britain. Microstructures and quartz c-axis analysis show that fabrics formed during nappe emplacement were thoroughly recrystallised before or during peak Caledonian metamorphic conditions. One segment of the shear zone was reactivated as a north-directed thrust with retrograde reworking of annealed metamorphic textures. Quartz-rich mylonites from the reactivated zone display asymmetric or single-girdle quartz c-axis fabrics consistent with north-directed overthrust shear, except within and immediately beneath an allochthonous orthogneiss sheet of Archaean basement. Here, quartz c-axis fabrics have orthorhombic symmetry, implying that the orthogneiss unit and the thin smear of psammitic mylonites accreted to its base extended coaxially as a rigid sheet while the softer quartz-rich mylonites of the enclosing Moine accommodated the late non-coaxial strain. The thrust zone was probably reactivated within the old footwall and by ductile extension of the hanging wall along localised zones of intense non-coaxial strain. This conclusion emphasises the complexity and longevity of Caledonian ductile deformation.

Development of preferred orientation and microstructure in sheared quartzite: comparison of natural data and simulated results

c-axis fabric and microstructures in a quartzite sample, sheared and extensively recrystallized under greenschist facies conditions, have been analyzed and compared with theoretical predictions using a viscoplastic self-consistent model modified to incorporate the effects of dynamic recrystallization. An asymmetric small-circle c-axis fabric about the finite shortening z-axis with a small half opening angle (35°) is present in the sample; it consists of four orientation components which are represented by host grain c-axis orientations (referred to as A, B, C and D): A and B are at high angles to the foliation plane, displaced against and with the sense of shear, respectively; C is in an intermediate direction between the Y- and Z-axis of finite strain, and D forms a subsidiary concentration around the intermediate strain ( Y-) axis. B- and C-grains are favorably oriented for basal (0001) and pyramid {10 1 ̄ 1}〈a〉 slip, respectively, and strongly deformed, while A- and D-grains are unfavorably oriented for the slip systems and little or moderately deformed. Some of A-grains are even fractured. The degree of dynamic recrystallization increases with increasing strain undergone by differently oriented grains (in the sequence of A-, D-, C- and B-grains). Microstructural evidence and theoretical predictions indicate that harder A-, C- and D-grains were significantly consumed by the grain boundary migration of the softer recrystallized B-component (although the consumption of A-grains was not really documented in the quartzite sample). The conclusion is supported by the fact that the B-component is much more dominant in the recrystallized than in the host c-axis fabric. Hence, the c-axis maximum nearly perpendicular to the shear plane and apparently displaced with the sense of shear commonly found in naturally sheared quartzites (correlated with the B-component) is presumably developed by the growth of soft orientations for basal (0001) slip by grain boundary migration at large strains.

C-Axis fabrics and microstructures in quartz schist from the Sambagawa metamorphic belt, central Shikoku, Japan

An integrated study on c-axis fabric and microstructure of plastically deformed and dynamically recrystallized quartz in quartz schist from the Sambagawa metamorphic belt in central Shikoku, Japan has revealed a correlation between c-axis fabrics and three-dimensional grain shape. Recrystallized quartz grains with prolate ( k > 1), oblate (0.2 < k < 1) and uniaxially oblate ( k < 0.2) shapes exhibit c-axis fabric patterns of cleft girdles around the principal finite elongation direction ( X), type I crossed girdles, and small circle girdles around the principal finite shortening direction ( Z), respectively. Fabric intensity calculated from the eigenvalues of orientation tensors of the c-axis fabrics increases with increasing magnitude of the apparent finite strain inferred from the shape of recrystallized quartz grains. These results indicate that the development of recrystallized grain shape as well as c-axis fabrics is primarily controlled by finite strain during progressive deformation. However, the finite strain estimated from three-dimensional grain shape is systematically deviated toward the flattening field compared to that estimated from the c-axis fabrics. We investigated several possible causes for the discrepancy, and propose that a scenario of coupled deformation and dynamic recrystallization in the quartz schist is responsible for the formation of these microstructures.

Influence of micaceous impurity on dynamically recrystallized quartz c-axis fabric in L- S tectonites from the Singhbhum Shear Zone and its footwall, Eastern India

When temperature and strain rate remain constant the quartz c-axis fabric in deformed pure quartz aggregates, is largely dependent on deformation kinematics. Asymmetry of the fabric, e.g. in type-I asymmetric crossed girdle pattern in natural quartz tectonites, simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non-coaxial flow. Natural quartz tectonites, however, often contain micaceous impurities. Measurement on a sample of 59 quartz tectonites with mesoscopic L- S fabric and representing low T T m , deformation under non-coaxial flow, from the Singhbhum Shear Zone and Dhanjori quartzites. Eastern India provides the basic data to quantitatively assess the influence of mica on (i) asymmetry of quartx c-axis fabric and (ii) degree of crystallographic preferred orientation, i.e. fabric intensity, taking c-axes of dynamically recrystallized quartz grains as a fabric element. A fabric intensity parameter (κ) is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c-axis orientations in each measured specimen. The modal percent of mica (μ) in the sample varies from 2 to 35; that of recrystallized quartz grains (ν), as opposed to relict clasts. from 45 to 98. The asymmetry of the fabrics in the above sample, measured either as the Am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content. Correlation-regression analysis of the variables κ, μ, and ν. demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter. The regression equation is of the form κ = 0.13 ν 1.11 μ −0.617.

Deformation mechanisms in the mylonite/ultramylonite transition

The deformation mechanisms and controls that operate in the mylonite/ultramylonite transition are interpreted from microstructural observation. The investigated mylonites and ultramylonites were derived from a granitic protolith which was deformed under greenschist facies conditions, and in the presence of fluid, in a regional-scale shear zone from northwest Argentina. Several deformation mechanisms were recognized to operate simultaneously in different domains of the microstructure at each particular stage of the microstructural evolution. This continuously mobile deformation partitioning, present throughout the microstructural evolution, ceases abruptly in the ultramylonite stage, where a stable-state microstructure is achieved. Domainal quartz c-axis fabrics indicate that quartz deforms by crystal-plastic processes at the initial and intermediate stages of deformation, but solution-transfer processes become predominant in the ultramylonite stage. Plagioclase is progressively transformed into muscovite through retrograde softening reactions. K-feldspar is progressively transformed into fine-grade aggregates via cataclastic flow and incipient recrystallization. Mica deforms by kinking and basal slip, with progressive development of fine-grained, morphologically oriented aggregates. Plagioclase disappearance as well as the development of intrafolial microfolds characterize the transition between the mylonitic and ultramylonitic domains. Disruption of these microfolds is interpreted to represent the ultimate control on the localization of the ultramylonite bands.

C-Axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid-rich greenschist conditions

The quartz v-axis orientations and microstructures have been analyzed in an originally single-crystal buckled quartz vein of greenschist metamorphic grade from the Sambagawa metamorphic belt, southwest Japan. The basal (0001) plane was inclined by ca 10–20 to the quartz vein surface before deformation, and the flexure (essentially kink) which produced the fold was accomplished by basal (0001) slip alone in the hinges (i.e. flexural-slip folding). In addition to crystal plasticity, dissolution microstructures are ubiquitous in the quartz vein. Dynamic recrystallization resulted from both subgrain rotation and grain-boundary migration. Rotation recrystallization associated with basal (0001) slip resulted in host-controlled v-axis orientation distribution along a great circle, whereas migration recrystallization, perhaps assisted by intergranular fluid, resulted in a penetrative development of a weak X (elongation)-maximum v-axis fabric nowhere rotated by the folding. The migration recrystallization is evidenced by large v-axis misorientations across the grain boundaries, as well as irregular grain shape. Based on the weak v-axis fabric development and lack of grain shape fabric, solution-precipitation creep may have dominated in the fluid-assisted grain-boundary migration regime, and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein.

Low-temperature deformation mechanisms at a lithotectonic interface near the Sudbury Basin, Eastern Penokean Orogen, Canada

Southwest of the Sudbury Basin, variations in macroscopic strain, microstructure and deformation mechanisms within Archaean granitoid basement and overlying Proterozoic rocks suggest that rock strength was strongly influenced by large amounts of fluids. Ductilely deformed granitoids with strong shape fabrics are concentrated in a 150–200-m-wide zone parallel to the subvertical basement-cover interface. High fluid activity in this zone is indicated by: (1) thickness and close spacing of foliation-parallel quartz veins, (2) abundant fluid-inclusion trails in quartz; (3) quartz c-axis fabrics: and (4) transformation of feldspar into phyllosilicates under middle greenschist-facies metamorphic conditions. Away from this zone, brittle-ductile deformation in Archaean granitoids caused grain size reduction of feldspar by fracturing and crystal-plastic deformation of quartz indicating largely ‘dry’ conditions. Strain was concentrated at the interface, in a region of bulk horizontal shortening and linked kinematically to large-scale shortening of the Sudbury Igneous Complex and thrusting in the South Range Shear Zone. After early open buckling of the Sudbury Igneous Complex, reverse shearing in this zone may have been controlled by ductile deformation at the basement-cover interface. Physical continuity of, and age constraints on, strain fabrics suggest that the South Range Shear Zone represents a segment of a large ductile deformation zone which is most likely linked to the Murray fault and on which higher-grade rocks ascended between 1.85 and 1.23 Ga ago.

Variations in shear deformation rate with depth at Dome Summit South, Law Dome, East Antarctica

The variation of shear deformation rate with depth at the Dome Summit South (DSS) site, 4.7 km (~4 ice thicknesses) from the summit of Law Dome, East Antarctica, has been determined by repeated borehole inclination measurement. The results show that from the surface down to 1000m (ice-equivalent depth! deformation rates increase as expected with the increase in stress, temperature and the development of stronger ice-crystal fabrics. There is a broad maximum in strain rate around 1000 m. Below this depth, strain rates decrease, with values in the basal ice ~1/3 of those at 1000 m. in DSS, Holocene ice with low, uniform impurity levels extends to a depth of 1110 m, so the decrease in shear rale below 1000 m is attributed not to any change in properties of the ice, but to shear stress reduction induced by the large-scale retarding effect of local bedrock hills. Below 1000 m, with in the zone of retarded flow, there is a narrow spike, 14 m thick, in which the shear rate is ~5 times that in the ice immediately above and below. The shear-rate spike corresponds in depth to ice with high dust concentrations, small crystal size and strong vertical c-axis fabrics that was deposited at the Last Glaciol Maximum. A surface velocity of 1.98 ± 0.03 m a−1 obtained by integration of shear rate over the borehole depth is in agreement with the value of 2.04 ± 0.11m a−1 obtained by the global positioning system. The ratio of average column velocity to surface velocity determined by the borehole measurements is 0.74. A value of 0.76 is obtained from mass-balance considerations.

Documentation of a 1450 Ma contractional orogeny preserved between the 1850 Ma Sudbury impact structure and the 1 Ga Grenville orogenic front, Ontario, Canada

A 1453–1445 Ma contractional deformation event is documented in rocks of the Early Proterozoic Huron Supergroup at the southeastern boundary between the Hudsonian craton to the northwest and an Early to Middle Proterozoic granitoid suite that forms the northeasternmost recognized extension of the Transcontinental Proterozoic Provinces. Detailed structural mapping, quartz c-axis fabrics, and grain-shape analysis indicate that four major orogenic phases have affected the area: (1) the Penokean orogeny, which predated the 1850 Ma Sudbury impact event, (2) ca. 1750 Ma granite plutonism, (3) ca. 1450 Ma orogenic events, and (4) the ca. 1000 Ma Grenvillian orogeny. The Murray fault zone, which likely accommodated considerable ductile displacement during the Penokean orogeny, underwent only brittle deformation during the younger orogenies. The previously undocumented Long Lake fault represents the northern limit of significant ductile south-over-north reverse thrusting during 1453–1445 Ma deformation. The effect of the Grenvillian orogeny in this area appears to have been minor. No deformation features could be linked to the intrusion of the ca. 1750 Ma granitoid suite.

Misalignment of quartz c-axis fabrics and lineations due to oblique final strain increments in the Ruby Mountains core complex, Nevada

To interpret c-axis stereonet patterns of deformed quartz the patterns must be viewed in the XZ strain plane, which is usually assumed to contain the elongation lineation. This study describes several c-axis patterns from a region of complex, amphibolite facies deformation in the northern Ruby Mountains where the late XZ plane is oblique to most of the lineations. Without the lineation as a guide, the proper orientation for interpretation of each pattern is determined by rotating the c-axis data around the normal to the foliation until the c-axes within the foliation plane lie at the center of the stereonet. This new view is inferred to be the XZ plane for the final increments of strain. The rotation produces an internal symmetry in the patterns. In addition, the rotated views across the area show a more consistent geographic orientation than the lineations. This geographic orientation is aligned with strain in a mylonitic shear zone which represents the latest macroscopic evidence for plastic strain along the edge of the study area. The complete reorientation of the quartz textures during the final strain increments implies that they record very little of the finite strain history. Quartz is the only mineral recrystallized during the late strain thereby providing insight into the geometry of strain increments that cannot be determined from other minerals.

Deformation microstructures of the quartz in quartz schist from the Sambagawa metamorphic belt in central Shikoku

In the Asemi River region of the Sambagawa metamorphic belt, central Shikoku (Fig.1), dozens of quartz schist layers are intercalated, in which well-defined quartz c-axis fabrics penetratively developed. In this pictorial, we show the close relationship between microstructures and c-axis fabrics in the deformed quartz, which indicates that both the microstructures reflect the deformation geometries caused by ductile flow in the Sambagawa metamorphic belt.The geological structure in the region is characterized by E-Wtrending and Ndipping foliation (Sb), and horizontal mineral lineation (Lm). ThesamplesforFigs.2, 3, 4 and 5 are collected respectively from the metamorphic zones as shown in Fig.1. In all the figures, X, Y and Z directions denote the directions parallel to Iineation, parallel to foliation and normal to lineation, and normal to foliation, respectively, and thesedirections can be approximated as the principal directions of strain ellipsoid(X>Y>Z).

Quartz C-axis fabric along Main Central Thrust (MCT), Garhwal Himalaya, India

The granite-gneisses are thrust over the less metamorphosed quartzites along the Main Central Thrust (MCT) in Garhwal Himalaya. The quartz C-axes fabrics from the plastically deformed quartzites and granite-gneisses were studied with the help of universal stage in XZ sections. The measurements of quartz C-axes in quartzite exhibit single and cross girdles. The C-axes maxima are located at the peripheries and towards the Y-axis. The quartz C-axes maxima in granite-gneisses are characterised by diffuse cross girdles with unequally populated pattern. The concentration of the quartz C-axes maxima is recorded progressively towards Y-axis.&#x0D; The single inclined girdles are generally exhibited by the quartz grains due to simple shear. The cross girdle patterns are due to conjugate direction of shear acting simultaneously under the influence of non-coaxial flattening (Bouchez and Pecher, 1981). The fabric of C-axes, commonly encountered in the shear zone are characterised by the maxima forming cross girdles which are asymmetric to the MCT transport direction. The quartz C-axes fabrics in quartzite formed by glide on the basal &lt;a&gt; and rhomb &lt;a&gt; systems. The deformation movements were of sinistral type during the formation of the MCT.

A structural record of the emplacement of the Pozanti-Karsanti ophiolite onto the Menderes-Taurus block in the late Cretaceous, eastern Taurides, Turkey

The Aladag region of eastern Taurides, Turkey, is characterized by an imbricated thrust structure developed during late stage emplacement of the Pozanti-Karsanti ophiolite onto the Menderes-Taurus block in the late Cretaceous. The mid to late Cretaceous dynamothermal metamorphic sole and the underlying unmetamorphosed mélange, here named the Aladag accretionary complex, were accreted to the base of the Pozanti-Karsanti ophiolite during intra-oceanic subduction, transport and final obduction of the ophiolite onto the Menderes-Taurus block. In the dynamothermal metamorphic sole, intensity of deformation and degree of metamorphism increase from the base to the top, and at least three episodes of foliation, lineation and fold development are recognized. The asymmetry of quartz c-axis fabrics, tightness and asymmetry of folds of the same generation, and curvature of fold hinge lines increase from base to top, indicating that non-coaxial progressive deformation prevailed during the development of the metamorphic sole. The mélange is divided into three major thrust fault-bounded tectonic slivers, each of which is characterized by distinctive types of matrix and block lithologies, structures and deformation style. Kinematic analyses of the dynamothermal metamorphic sole and the mélange reveal that the tectonic transport direction of the Pozanti-Karsanti ophiolite during its emplacement was from north-northwest to south-southeast, suggesting that the Pozanti-Karsanti ophiolite was derived from a Neo-Tethyan ocean to the north of the Menderes-Taurus block.

A new estimate of the conditions for transition from basal 〈 a〉 to prism [ c] slip in naturally deformed quartz

Quartz c-axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [ c], prism 〈 a〉 and basal 〈 a〉 slip systems were activated during the plastic deformation under high-temperature metamorphic conditions. With increasing temperature, quartz c-axis fabric patterns change from girdle type, through X-maximum with a faint crossed girdle, to X-maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature, closely correlated with the change in quartz c-axis fabric. The fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size. Moreover, observations with transmission electron microscopy (TEM) clearly identify the dominant activation of [ c] slip through the presence of basal subgrain boundaries and free dislocations with the [0001] Burgers vector in the quartz from the metachert samples exhibiting X-maximum c-axis fabric. Based on these observations it is concluded that the mechanism switch from basal 〈 a〉 to prism [ c] slip systems occurred with increasing temperature in the metacherts. The transition temperature is petrologically estimated to be ca. 550–600°C at the natural strain rate.

Mapping c-axis fabrics to study physical processes in ice : R. B. Alley, A. J. Gow & D. A. Meese, Journal of Glaciology, 41(137), 1995, pp 197–203

Mapping c-axis fabrics to study physical processes in ice : R. B. Alley, A. J. Gow & D. A. Meese, Journal of Glaciology, 41(137), 1995, pp 197–203

Retrograde evolution of quartz segregations from the Dos Picos shear zone in the Nevado-Filabride Complex (Betic chains, Spain). Evidence from fluid inclusions and quartz c-axis fabrics

Synkinematic quartz veins are ubiquitous in the shear zone separating the Veleta unit from the Calar Alto unit in the internal part of the Betic Cordilleras. They have been studied with respect to quartz c-axis fabrics, microstructures and fluid inclusions. Veins were probably generated during syn-metamorphic stacking of the units at P = 500 − 600 MPa and T = 400 − 500°C. Quartz displays two groups of microstructures in the shear zone: (1) older coarse-grained mosaics (CGM) resulting from exaggerated grain growth; and (2) younger fine-grained mosaics (FGM) developed at the expense of the former. The fine-grained mosaics show polygonal granoblastic and elongate mosaic microstructures in general, with ribbon microstructures often found near the boundary of the units. Fluids contained in secondary inclusions vary from high salinity brines to different types of CO2—brine mixtures and low density CO2 fluids. Differences in composition and P-T trapping conditions are indicated for the different types of inclusions. Some fluid inclusions are older than the FGM, whereas others are younger, thus constraining the P- T conditions at which the two microstructural events took place. Fluid inclusion evidence suggests conditions of Pfluid > 170 MPa and T ≧ 370−430°C for the CGM and Pfluid ≧ 20−80 MPa and T > 340°C for the FGM. The quartz c-axis fabrics dealt with here correspond to the second recrystallization event, as little evidence of older fabrics is preserved in the shear zone. C-axis patterns vary across the shear zone from slightly asymmetrical type I crossed girdles in the hanging wall and footwall to more asymmetrical crossed girdles at the boundary of the units. This indicates a correlative increase in the magnitude of the heterogeneous shear strain in the same direction. Most of the deformation is concentrated at the top of the Veleta unit. The sense of movement is top to the west, in agreement with other kinematic markers. The quartz c-axis fabrics resulted from dynamic recrystallization during simple shear. The retrograde P-T path inferred from fluid inclusion analysis, along with other geological and geochronological evidence, indicates that this deformation is coeval with a reduction in the crustal overburden. Geochronological and stratigraphic data show that the proposed Dos Picos extensional detachment, separating the Calar Alto and Veleta units, took place during the early Miocene, synchronous with the intense thinning of the Nevado-Filabride Complex and of the whole continental crust underlying the Alboran Basin.

Syn-collisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics: the Salamanca Detachment Zone (Central Iberian Zone, Spain)

The area south of the town of Salamanca is a zone of the Spanish Variscan Belt characterized by a first compressional event which gave rise to steep structures, followed by the development of an extensional ductile shear zone, more than 4 km thick, that is described as the Salamanca Detachment Zone. The strain associated to the detachment is analyzed using quartz pebbles in conglomerates. Comparing the measured strain values with theoretical strain paths, leads to the conclusion that the deformation was approximately of plane strain type. Quartz c-axis fabrics and kinematic criteria indicate that simple shear was a very important component of the deformation, though coaxial components were probably involved. The extensional character of the detachment is indicated by the geometry of the metamorphic zones and the metamorphic evolution. Taken together, the data indicate that the extensional event was related to gravitational collapse induced by the thickening of the continental crust, and that it was syn-collisional. The movement of the hangingwall unit was to the southeast, parallel to the trend of the fold belt, and the translation was of the order of one to a few dozens of km. Though important, this is not exceedingly large, and allowed the preservation of low-grade metamorphic conditions in the hangingwall, preventing it from being affected by extensive brittle tectonic processes.