Complex flow in lowest crustal, anastomosing mylonites: Strain gradients in a Kohistan gabbro, northern Pakistan

International audienceStrain localization in near paleo-Moho metagabbros of the Kohistan Arc, northern Pakistan, produced anastomosing shear zones. The two-dimensional (2-D) analysis of strain gradients in planes parallel to the general flow direction reveals simple shear strain >7 combined with about 50% surface loss. The pargasite-garnet assemblage of the mylonite has a density 2.8% higher than the protolith, which is insufficient to account for the measured surface loss. Furthermore, major and trace element compositions of the gabbro and the mylonite indicate isochemical deformation. Average V p and acoustic impedance measured at room temperature and up to 300 MPa increase from gabbro through the gradient zone to mylonite; they are consistent with density measurements. The three-dimensional analysis of shape-preferred orientation and lattice-preferred orientation of sheared and synkinematic minerals indicates that the 2-D surface loss reflects sideways and lengthwise material transfer. Sideways and lengthwise material transfers take place in widening and lengthening mylonites, respectively. The anastomosing shear zone pattern impels this complexity of the regional flow. We conclude that shear zones with a thinning component are likely representative of deep crustal flow in other tectonic environments

Mesozoic-Tertiary structural evolution of an extensional gneiss dome—the Kesebir-Kardamos dome, eastern Rhodope (Bulgaria-Greece)

The tectonic evolution of the Rhodope massif involves Mid-Cretaceous contractional deformation and protracted Oligocene and Miocene extension. We present structural, kinematic and strain data on the Kesebir-Kardamos dome in eastern Rhodope, which document early Tertiary extension. The dome consists of three superposed crustal units bounded by a low-angle NNE-dipping detachment on its northern flank in Bulgaria. The detachment separates footwall gneiss and migmatite in a lower unit from intermediate metamorphic and overlying upper sedimentary units in the hanging wall. The high-grade metamorphic rocks of the footwall have recorded isothermal decompression. Direct juxtaposition of the sedimentary unit onto footwall rocks is due to local extensional omission of the intermediate unit. Structural analysis and deformational/metamorphic relationships give evidence for several events. The earliest event corresponds to top-to-the SSE ductile shearing within the intermediate unit, interpreted as reflecting Mid-Late Cretaceous crustal thickening and nappe stacking. Late Cretaceous-Palaeocene/Eocene late-tectonic to post-tectonic granitoids that intruded into the intermediate unit between 70 and 53Ma constrain at least pre-latest Late Cretaceous age for the crustal-stacking event. Subsequent extension-related deformation caused pervasive mylonitisation of the footwall, with top-to-the NNE ductile, then brittle shear. Ductile flow was dominated by non-coaxial deformation, indicated by quartz c-axis fabrics, but was nearly coaxial in the dome core. Latest events relate to brittle faulting that accommodated extension at shallow crustal levels on high-angle normal faults and additional movement along strike-slip faults. Radiometric and stratigraphic constraints bracket the ductile, then brittle, extensional events at the Kesebir-Kardamos dome between 55 and 35Ma. Extension began in Paleocene-early Eocene time and displacement on the detachment led to unroofing of the intermediate unit, which supplied material for the syn-detachment deposits in supra-detachment basin. Subsequent cooling and exhumation of the footwall unit from beneath the detachment occurred between 42 and 37Ma as indicated by mica cooling ages in footwall rocks, and extension proceeded at brittle levels with high-angle faulting constrained at 35Ma by the age of hydrothermal adularia crystallized in open spaces created along the faults. This was followed by Late Eocene-Oligocene post-detachment overlap successions and volcanic activity. Crustal extension described herein is contemporaneous with the closure of the Vardar Ocean to the southwest. It has accommodated an earlier hinterland-directed unroofing of the Rhodope nappe complex, and may be pre-cursor of, and/or make a transition to the Aegean back-arc extension that further contributed to its exhumation during the Late Miocene. This study underlines the importance of crustal extension at the scale of the Rhodope massif, in particular, in the eastern Rhodope region, as it recognizes an early Tertiary extension that should be considered in future tectonic models of the Rhodope and north Aegean region

Tectonique et microtectonique des séries cristallophylliennes du Haut-Allier et de la vallée de la Truyère : contribution du microscope électronique à transmission à l'étude de la déformation des minéraux dans les zones profondes

This work identified two petrologically different units, from top to bottom. (1) A retrogressed unit, initially granulitic, now represented by anatexites and the leptyno-amphibolite group; this unit has undergone a regional amphibolite facies retrogression. (2) A prograde unit, initially a grauwacko-pelitic series now represented by biotite-sillimanite gneiss in which no sign of retrogression was found. Between these two units, a sillimanite-rich gneiss sequence is underlain by peridotites, and the sillimanite concentration is related to intense shear. Arguments are exposed to consider that the retrograde unit is a basement thrust over the prograde unit, which delivered retrogressing fluids while being metamorphosed: the inverse superposition of the two units; granulites retrogressed to anatexites while the underlying series underwent prograde metamorphism; the large-scale thrust tectonics in which all the series were involved. A synthetic cross-section (fig. 23) and structural map (fig. 24) summarize this conclusion. The few available geochronological measurements point to a major Hercynian orogenic event. The second part of this work is devoted to the physics of shear strain. Transmission electron microscopy reveals the importance of intracrystalline deformation. This allows using lattice preferred orientations, especially those of quartz c-axes, as a criterion for shear direction, and thus the transport direction of the geological units deformed in a ductile manner.Deux ensembles pétrologiques différents sont distingués, de haut en bas. (1) UN ENSEMBLE RÉTROGRADE, initialement granulitique, qui a subi une rétromorphose régionale dans le faciès amphibolite et est maintenant représenté par les anatexites à cordiérites et le groupe leptyno-amphibolique; (2) UN ENSEMBLE PROGRADE, représenté par les gneiss à biotite et sillimanite, ex-série grauwacko-pélitique, dans lesquels aucun signe de rétromorphose n’a été trouvé. Entre ces deux ensembles, un terme intermédiaire riche en sillimanite, est jalonné à sa base par des péridotites (en particulier tout autour de l'unité de Massiac). La concentration de la sillimanite y serait favorisée par un cisaillement important. La rétromorphose de l’ensemble [1] nécessite des fluides chimiquement actifs. Deux solutions sont envisagées: (a) L'eau provient du manteau (ce qui traduit une importante activité hydrothermale); (b) Ces roches de faciès granulite [1] sont venues chevaucher des séries riches en eau [2]. Les arguments pour considérer que l’ensemble rétrograde est un socle charrié sur l'ensemble prograde sous-jacent sont : la présence d'un ensemble ayant eu une histoire granulitique puis rétromorphosé quand la série sous-jacente a subi son premier métamorphisme prograde; la superposition inverse des deux grands ensembles; l'importante tectonique tangentielle dans laquelle toutes les séries sont impliquées; la présence dans le Rouergue de séries granulitiques [1] situées normalement sous une même série grauwacko-pélitique [2]. C'est dans ce dernier matériel que l'on doit trouver l'origine des fluides de rétromorphose, la tectonique liée au transport chevauchant favorisant l'invasion du socle chevauchant par les fluides. La coupe synthétique de la rive droite de l'Allier (fig. 23) et le modèle structural proposé (fig. 24) résument cette conclusion à l’échelle régionale. Les rares mesures géochronologiques disponibles permettent d’attribuer la mise en place des nappes rétrogrades et le métamorphisme régional prograde dans les séries grauwacko-pélitiques et rétrograde dans les granulites à l'orogenèse hercynienne. LA DEUXIÈME PARTIE DE CE TRAVAIL est consacrée à la physique de la déformation cisaillante identifiée dans les gneiss du Massif Central. La microscopie électronique en transmission met en évidence l’importance de la déformation intracristalline. Ceci permet d’utiliser les orientations préférentielles de réseau, en particulier celle des axes c du quartz, comme critère de sens de cisaillement, et donc de la direction de transport des unités géologiques déformées de façon ductile

Development of a seismic source model for probabilistic seismic hazard assessment of nuclear power plant sites in Switzerland: the view from PEGASOS Expert Group 4 (EG1d)

Abstract.: We present a seismogenic source model for site-specific probabilistic seismic hazard assessment at the sites of Swiss nuclear power plants. Our model is one of four developed in the framework of the PEGASOS project; it contains a logic tree with nine levels of decision-making. The two primary sources of input used in the areal zonation developed by us are the historical and instrumental seismicity record and large-scale geological/rheological units. From this, we develop a zonation of six macrozones, refined in a series of seven decision steps up to a maximum of 13 zones. Within zones, activity rates are either assumed homogeneous or smoothed using a Gaussian kernel with width of 5 or 15km. To estimate recurrence rate, we assume a double truncated Gutenberg-Richter law, and consider five models of recurrence parameters with different degrees of freedom. Models are weighted in the logic tree using a weighted Akaike score. The maximum magnitude is estimated following the EPRI apporach. We perform extensive sensitivity analyses in rate and hazard space in order to assess the role of declustering, the completeness model, quarry contamination, border properties, stationarity, regional b-value and magnitude-dependent hypocentral dept

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite-migmatite complex with the Bergell pluton

We describe field occurrences of sapphirine-bearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of sub-parallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740°C and 6.5-7.5kbar in Oligocene times (ca 30Ma). Metamorphic conditions in the charnockites and granulites (>920°C for 8.5-9.5kbar) largely exceed these regional metamorphic conditions and are dated at 282-260Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf comple

Exo-zodiacal disk mapper: a space interferometer to detect and map zodiacal disks around nearby stars

We propose a concept for a space mission designed to make a survey of potential zodiacal dust disks around nearby stars in the mid-IR. We show that a 10-meter baseline nulling interferometer with two 0.6-meter apertures located in a 1 X 1 AU heliocentric orbit would allow for the survey of about 400 stars in the solar neighborhood and permit a first order determination of the disk inclination and of the dust density and temperature radius dependence. The high dynamic range of the instrument may also be used to study an additional astrophysical phenomena. Beyond its own scientific merit, such a mission would also serve as a technological precursor to a larger interferometer of the type being considered for the detection of earth-like planets

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.National Science Foundation (U.S.) (Grant EAR 6920005

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the main rock units of the Chilas Complex, a series of lower crustal intrusions emplaced during initial rifting within the Mesozoic Kohistan (paleo)-island arc (NW Pakistan). Detailed field observations and petrological analysis, together with geochemical data, indicate that the two principal units, ultramafic rocks and gabbronorite sequences, originate from a common parental magma, but evolved along different mineral fractionation trends. Phase petrology and mineral trace element data indicate that the fractionation sequence of the ultramafic rocks is dominated by the crystallization of olivine and clinopyroxene prior to plagioclase, whereas plagioclase precedes clinopyroxene in the gabbronorites. Clinopyroxene in the ultramafic rocks (with Mg-number [Mg/(Fetot + Mg] up to 0·95) displays increasing Al2O3 with decreasing Mg-number. The light rare earth element depleted trace element pattern (CeN/GdN ∼0·5-0·3) of primitive clinopyroxenes displays no Eu anomaly. In contrast, clinopyroxenes from the gabbronorites contain plagioclase inclusions, and the trace element pattern shows pronounced negative anomalies for Sr, Pb and Eu. Trace element modeling indicates that in situ crystallization may account for major and trace element variations in the gabbronorite sequence, whereas the olivine-dominated ultramafic rocks show covariations between olivine Mg-number and Ni and Mn contents, pointing to the importance of crystal fractionation during their formation. A modeled parental liquid for the Chilas Complex is explained in terms of mantle- and slab-derived components, where the latter component accounts for 99% of the highly incompatible elements and between 30 and 80% of the middle rare earth elements. The geochemical characteristics of this component are similar to those of a low percentage melt or supercritical liquid derived from subducted mafic crust. However, elevated Pb/Ce ratios are best explained by additional involvement of hydrous fluids. In accordance with the crystallization sequence, the subsolidus metamorphic reactions indicate pressures of 0·5-0·7 GPa. Our data support a model of combined flux and decompression melting in the back-ar