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Papers by Pavlina Hasalová

In the Central Vosges (France), we describe and model the structural relationships existing betwe... more In the Central Vosges (France), we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites. These domains show a structural and anisotropy of magnetic susceptibility (AMS) zonation associated with an increasing amount of melt from the core to their margins. A pre-extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy. The internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss/granite multilayer. The external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt-orthogneiss multilayer, pinchand-swell structures and extensional kink bands along which granite veins have been injected. The surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher. This systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss. Numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite AMS fabrics in migmatites.

Quartz crystallographic preferred orientations (CPO) from three distinct orthogneisses using both... more Quartz crystallographic preferred orientations (CPO) from three distinct orthogneisses using both the Electron Back Scatter Diffraction (EBSD) and Fabric Analyser (FA) techniques reveal a clear trend from basal and rhomb <a þ c> slip for high PeT conditions (670 AE 20 C/9 kbar), rhomb <a þ c> and basal slip for medium PeT (590 AE 15 C/6 kbar) and a dominance of prism slip for lower PeT conditions (<570 C/4e5 kbar). The textural variations are interpreted in terms of a temperature field gradient and microscale strain partitioning controlled by a weak feldspar matrix that can locally invert the expected slip system sequences. Locally quartz CPOs are different within one thin section, and in comparison to bulk orientation measurements both, EBSD and the Fabric Analyser techniques are ideal to determine such textural heterogeneities. While the EBSD is a powerful technique to determine the full CPO, measurements from similar locations inside several quartz grains from the orthogneisses and an annealed and undeformed quartzite show that the FA is an accurate tool to determine CPOs of c-axis orientations in uniaxial materials. In a CPO focussed study the FA is a cheap alternative to EBSD as the analysis of whole thin section can be accomplished in a very short time, with minimal sample preparation. With the FA it is possible to evaluate the CPOs of several samples quickly with an accuracy that allows identification of the main slip systems and their homogeneity.

... of magma, the system did not lose continuity, suggesting that it did not flow en masse ... PA... more ... of magma, the system did not lose continuity, suggesting that it did not flow en masse ... PANGONG RANGE: ANATECTIC TERRANE The Pangong Range is an exhumed wedge of rocks in between ... was likely to be a result of magma from different sources flowing through the same ...

ABSTRACT Migmatite terranes are structurally complex because of strong rheological contrast betwe... more ABSTRACT Migmatite terranes are structurally complex because of strong rheological contrast between layers with different melt contents and because of magma migration leading to irregular volume changes. Migmatite deformation is intimately linked with magma extraction and the origin of granitoids, and is an important process at the root of arcs and mountain belts. We investigate here the relationships between an evolving deformation and magma extraction in migmatites formed during the ca. 500Ma Delamerian orogeny, exposed on Kangaroo Island, South Australia. Here, several phases of deformation occurred in the presence of melt. During an early upright, non-cylindrical folding event, magma was channeled towards the hinge zones of antiforms. Funnel-shaped networks of leucosomes form a root zone that link up towards a central axial planar channel, forming the main magma extraction paths during folding. Extraction was associated with fold limb collapse, and antiformal hinge disruption by magma accumulation and transfer. During a later deformation phase, melt-rich diatexites were deformed, and schollen were disaggregated into smaller blocks and schlieren, and deformed into asymmetric, sigmoidal shapes indicative of dextral shearing flow. During flow, magma accumulated preferentially along shear planes, indicating a dilatational component during shearing (transtension) and on strain shadows of schollen. As deformation waned, magma extraction from these diatexites gave rise to N-trending, steeply dipping, funnel-shaped channels not associated to any deformational feature. The funnel-shape of these structures indicates the direction of magma flow. Structures developed during this phase are comparable with those formed during dewatering of soft sediments. Despite a high degree of complexity, magma migration and extraction features record distinct responses to the evolving deformation which can be used to understand deformation, and the nature of magma extraction and its direction. The oldest and youngest magmatic rocks from migmatites were dated (U-Pb monazite, SHRIMP). Both reveal continuous age spread of ~470−490Ma with two dominant age groups of ~470Ma and ~485Ma corresponding to monazite rims and cores, respectively. The age range is interpreted to indicate the duration of anatexis (order of 20Ma) with the two peaks marking individual magma batch crystallization during D1 and D4 deformation event.

ABSTRACT The Zanskar shear zone is a ductile, normal-sense shear zone that exploited the contact ... more ABSTRACT The Zanskar shear zone is a ductile, normal-sense shear zone that exploited the contact between the High Himalayan Crystalline series and the Tethyan sedimentary series. The Zanskar shear zone is an extension of the South Tibetan detachment system with similar timing and nature, and, in Zanskar, it accommodated 24 km of normal movement. Early thrusting is preserved in the footwall and hanging wall and is overprinted by normal shearing. Thrusting and normal shearing were coplanar and codirectional, with SW-directed thrusting overprinted by NE-directed normal shearingâ€”a simple inversion of movement sense. The telescoped isograds related to normal shearing define a broad pattern of colder rocks on top of hotter. However, we found preserved thrust-related metamorphic series, with hotter rocks on top of colder, severely telescoped by normal shearing. Some determinations of the amount of displacement and thinning on the Zanskar shear zone prior to the current work have assumed a steady-state crustal profile and have disregarded preexisting perturbations of isograds such as those indicated here. Miocene leucogranitic intrusions accumulated within and below the normal Zanskar shear zone. Intrusions were sheared during thrusting and normal movement, and magmatism outlasted normal shearing. We have dated monazites by U-Pb sensitive high-resolution ion microprobe (SHRIMP) from leucogranite samples that were sheared by the thrusting event, by the normal movement event, and those that postdate all shearing. Results constrain the timing of the switch from thrusting to normal movement to between 26 and 24 Ma and ca. 22 Ma. At ca. 20 Ma, normal shearing in the eastern Zanskar shear zone was no longer active, and magmatism was waning, producing late, undeformed leucogranitic dikes. Taking into account the shear zone thickness of 0.83 km, the maximum duration of normal movement of 6 m.y., and the estimated strain of Î³ = 28.6, we estimate the lower bound of strain rate for the Zanskar shear zone to be 1.5 Ã— 10âˆ’13 sâˆ’1. Given the short duration of the normal shearing event and magmatism, we find little support for the hypothesis of channel flow in Zanskar. We propose instead that Miocene anatexis weakened the midcrustal levels and was the cause of the switch from thrusting to normal fault movement, doming, and cooling of the anatectic core of the High Himalayan Crystalline series.

Despite the fact that the number of officially classified meteorites is now over 45,000, we lack ... more Despite the fact that the number of officially classified meteorites is now over 45,000, we lack a clearly defined sequence of samples from a single parent body that records the entire range in metamorphic temperatures from pristine primitive meteorites up to the temperatures required for extensive silicate partial melting. Here, we conduct a detailed analysis of Watson 012, an H7 ordinary chondrite, to generate some clarity on the textural and chemical changes associated with equilibriumbased silicate partial melting in chondritic meteorites. To do this we compare the textures in the meteorite with those preserved in metamorphic contact aureoles on Earth. The most distinctive texture generated by the partial melting that affected Watson 012 is an extensively interconnected plagioclase network, which is clearly observable with a petrographic microscope. Enlarged metal-troilite grains are encapsulated at widenings in this plagioclase network, and this is clearly visible in reflected light. Together with these features, we define a series of other characteristics that can be used to more clearly classify chondritic meteorites as being of petrologic Type 7. To provide comprehensive evidence of silicate partial melting and strengthen the case for using simple petrographic observations to classify similar meteorites, we use high-resolution X-ray computed tomography to demonstrate that the plagioclase network has a high degree of interconnectedness and crystallised as large (cm-scale) skeletal crystals within an olivine-orthopyroxene-clinopyroxene framework, essentially pseudomorphing a melt network. Back-scattered electron imaging and element mapping are used to show that some of the clino-and orthopyroxene in Watson 012 also crystallised from silicate melt, and the order of crystallisation was orthopyroxene ! clinopyroxene ! plagioclase. X-ray diffraction data, supported by bulk geochemistry, are used to show that plagioclase and ortho-and clinopyroxene were added to the Watson 012 sample by through-flowing basaltic melt. Along with the absence of glass and granophyre, this interconnected network of coarse-grained skeletal plagioclase indicates that the sample cooled slowly at depth within the parent body. The evidence of melt flux indicates that Watson 012 formed in the presence of a gravitational gradient, and thus at significant distance from the centre of the H chondrite parent body (the gravitational gradient at the centre would be zero). Our interpretation is that incipient silicate partial melting in Watson 012 occurred when a region of radiogenically heated H6 material located at considerable depth (possibly at $15-20 km from surface) was heated by an additional ca. 200-300°C in association with a large shock event. Due to insulation at depth within an already hot parent body, the post-shock temperature equilibrated and remained above the solidus long enough for widespread equilibrium-based silicate partial melting, and for melt to migrate. Although the observed melting may have been facilitated by additional heating http://dx.

Gondwana Research, 2014

The Trans-Altai Zone in southern Mongolia is characterized by thrusting of greenschist-facies Sil... more The Trans-Altai Zone in southern Mongolia is characterized by thrusting of greenschist-facies Silurian oceanic rocks over Devonian and Lower Carboniferous volcano-sedimentary sequences, by E-W directed folding affecting the early Carboniferous volcanic rocks, and by the development of N-S trending magmatic fabrics in the Devonian-Carboniferous arc plutons. This structural pattern is interpreted as the result of early Carboniferous thick-skinned E-W directed nappe stacking of oceanic crust associated with syn-compressional emplacement of a magmatic arc. The southernmost South Gobi Zone represents a Proterozoic continental domain affected by shallow crustal greenschist-facies detachments of Ordovician and Devonian cover sequences from the Proterozoic substratum, whereas supracrustal Carboniferous volcanic rocks and Permian sediments were folded into N-S upright folds. This structural pattern implies E-W directed thin-skinned tectonics operating from the late Carboniferous to the Permian, as demonstrated by K-Ar ages ranging from~320 Ma to 257 Ma for clay fractions separated from a variety of rock types. Moreover, the geographical distribution of granitoids combined with their geochemistry and SHRIMP U-Pb zircon ages form distinct groups of Carboniferous and Permian age that record typical processes of magma generation and increase in crustal thickness. The field observations combined with clay ages, the geochemical characteristics of the granitoids and their ages imply that the E-W trending zone affected by tectonism migrated southwards, leaving the Trans Altai Zone inactive during the late Carboniferous and Permian, suggesting that the two units were tectonically amalgamated along a major E-W trending strike slip fault zone. This event was related to late Carboniferous subduction that was responsible for the vast volume of granitoid magma emplaced at 300-305 Ma in the South Gobi and at 307-308 Ma in the Trans-Altai Zones. The formation and growth of the crust was initially due only to subduction and accretion processes. During the post-collisional period from 305 to 290 Ma the addition of heat to the crust led to the generation of (per-) alkaline melts. Once amalgamated, these two different crustal domains were affected by N-S compression during the Triassic and early Jurassic (185-173 Ma), resulting in E-W refolding of early thrusts and folds and major shortening of both tectonic zones.

Journal of Structural Geology, 2009

Gondwana Research, 2014

Journal of Structural Geology, 2009

Gondwana Research, 2014