Main Shear Zone Research Articles

Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain):in situ40Ar/39Ar ages andP–T–d–tpaths for the Nevado‐Filabride complex

AbstractThe combination of metamorphic petrology tools andin situlaser40Ar/39Ar dating on phengite (linking time of growth, compositions andP–Tconditions) enables us to identify a detailedP–T–d–tpath for the still debated tectonometamorphic evolution of the Nevado‐Filabride complex and infer new geodynamic‐scale constraints. Our data show an isothermal decompression (at 550 °C) from 20 kbar for the Bédar‐Macael unit and 14 kbar for the Calar Alto unit down toc.3–4 kbar for both units at 2.8 mm year−1. At 22–18 Ma, this first part of the exhumation is followed by a final exhumation at 0.6 mm year−1along a high‐temperature low‐pressure (HTLP) gradient ofc. 60 °C km−1. The age of the peak of pressure is not precisely known but it is shown that it is around 30 Ma and possibly older, which is at variance with recent models suggesting a younger age for high‐pressure (HP) metamorphism. Most of the exhumation is related to late‐orogenic extension fromc. 30 to 22–18 Ma. Thus the formation of the main ductile extensional shear zone, the Filabres Shear Zone (FSZ), occurred at 22–18 Ma and is clearly associated with a top‐to‐the‐west shear sense once the FSZ is well localized. The transition from ductile to brittle then occurred atc. 14 Ma. The final exhumation, accommodated by brittle deformation, occurred fromc. 14 to 9 Ma and was accompanied, from 12 to 8 Ma, by the formation of nearby extensional basins. The duration of the extensional process isc.20 Myr which argues in favour of a progressive slab retreat fromc. 30 to 9 Ma. The change in the shape of theP–Tpath at 22–18 Ma together with strain localization along the main top‐to‐the‐west shear zone suggests that this date corresponds to a change in the direction of slab retreat from southwards to westwards.

Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary

Research Article| June 01, 2005 Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary James E. Faulds; James E. Faulds 1Nevada Bureau of Mines and Geology, MS 178, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Christopher D. Henry; Christopher D. Henry 1Nevada Bureau of Mines and Geology, MS 178, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Nicholas H. Hinz Nicholas H. Hinz 1Nevada Bureau of Mines and Geology, MS 178, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Geology (2005) 33 (6): 505–508. https://doi.org/10.1130/G21274.1 Article history received: 07 Oct 2004 rev-recd: 22 Jan 2005 accepted: 05 Feb 2005 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James E. Faulds, Christopher D. Henry, Nicholas H. Hinz; Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary. Geology 2005;; 33 (6): 505–508. doi: https://doi.org/10.1130/G21274.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In the western Great Basin of North America, a system of dextral faults accommodates 15%–25% of the Pacific–North American plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this system. This young evolving part of the plate boundary offers insight into how strike-slip fault systems develop and may reflect the birth of a transform fault. A belt of overlapping, left-stepping dextral faults dominates the northern Walker Lane. Offset segments of a W-trending Oligocene paleovalley suggest ∼20–30 km of cumulative dextral slip beginning ca. 9–3 Ma. The inferred long-term slip rate of ∼2–10 mm/yr is compatible with global positioning system observations of the current strain field. We interpret the left-stepping faults as macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. The strike-slip faults end in arrays of ∼N-striking normal faults, suggesting that dextral shear diffuses into extension in the Great Basin. Coeval extension and dextral shear have induced slight counterclockwise fault-block rotations, which may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating development of a throughgoing strike-slip fault. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north‐east India

AbstractIn the Sikkim region of north‐east India, the Main Central Thrust (MCT) juxtaposes high‐grade gneisses of the Greater Himalayan Crystallines over lower‐grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large‐scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th–Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14–15 and 12–10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14–15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25 °C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25 °C and 6 ± 1 kbar. The P–T–t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid‐crustal extrusion and the role of out‐of‐sequence thrusts in convergent plate tectonic settings.

Uppermost Tortonian to Quaternary depocentre migration related with segmentation of the strike-slip Palomares Fault Zone, Vera Basin (SE Spain)

The Palomares Fault Zone (PFZ) is one of the main strike-slip brittle shear zones found in the Betics. It is segmented in several faults that have been active between the Upper Tortonian and present day. Data from drill cores in the Palomares area have permitted us to define the geometry and location of sedimentary depocentres related with the PFZ. These data show an eastward displacement between the Upper Tortonian to Messinian and the Pliocene–Quaternary sedimentary depocentres, towards the presently active Arteal fault, which bounds the western mountain front of Sierra Almagrera, showing that deformation along this fault zone has migrated towards the east, from the Palomares segment, with its main activity during the Upper Tortonian and Messinian, towards the Arteal fault, active during the Pliocene and Quaternary. To cite this article: G. Booth-Rea et al.,

Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO 2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at ∼0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zone’s major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnäsite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO 3 2−, PO 4 2−, and SO 4 2−ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.

Mode of lithospheric extension: Conceptual models from analogue modeling

Comparison of analogue experiments at crustal and lithospheric scale provides essential information concerning the mode of deformation during lithospheric extension. This study shows that during extension, lithospheric deformation is controlled by the development of shear zones in the ductile parts. At lithospheric scale, the global deformation is initiated by the rupture of the brittle mantle lithosphere. This failure generates the formation of conjugate and opposite shear zones in the lower crust and the ductile mantle lithosphere. The analysis of the internal strain of the ductile layers suggests that the two opposite shear zones located below the asymmetric graben in the lower crust and the ductile mantle lithosphere prevail. Experiments show that from a similar initial stage, the relative predominance of these shear zones originates two different modes of deformation. If the crustal shear zone prevails, a major detachment‐like structure crosscuts the whole lithosphere and controls its thinning. In this model named the simple shear mode, the resulting geometry shows that crustal and lithospheric thinning are laterally shifted. If the mantle shear zone predominates, the lithospheric thinning is induced by the coeval activity of the two main shear zones. This process called the necking mode leads to the vertical superposition of crustal and mantle lithospheric thinning. Applied to natural laboratories (West European rift, Red Sea rift and North Atlantic), this conceptual model allows a plausible explanation of the different geometries and evolutions described in these provinces. The North Atlantic and the Red Sea rift systems may result from a simple shear mode, whereas the necking mode may explain part of the evolution of the West European rift especially in the Massif Central and the Eger graben.

Large extensional structures developed during emplacement of a crystalline thrust sheet: the Mondoñedo nappe (NW Spain)

The Mondoñedo nappe is a crystalline thrust sheet characterized by large recumbent folds, regional intermediate-pressure metamorphism, synkinematic intrusion of granitoids during nappe emplacement, and an extensional ductile shear zone developed within the nappe during thrusting. A large tectonic window permits the study of the footwall unit, revealing another extensional shear zone contemporaneous with thrusting and a low-pressure metamorphic evolution, in contrast to that of the hanging wall unit. The two main extensional shear zones produced E–W extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend. Furthermore, subordinate N–S longitudinal extension was accommodated by normal faults in the footwall, and some of these faults were used as lateral ramps in late stages of thrusting. The role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate convergence but characterized by large-scale rheological heterogeneities within it. Deep-seated viscous flow, triggered by heat accumulation, seems to account for the horizontal stretching and probable tapering of the orogenic wedge, which was induced by gravitational instabilities due to partial melting and underplating by buoyant continental crust.

Low- P metamorphism following a Barrovian-type evolution. Complex tectonic controls for a common transition, as deduced in the Mondoñedo thrust sheet (NW Iberian Massif)

The Mondoñedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low- and medium- P metamorphic domains. This unit underwent an initial medium- P event during the initial stages of Variscan convergence, related to crustal thickening. Subsequently, the thrust sheet evolved to a low- P baric type of metamorphism, related to syn-convergence thinning and exhumation. Its footwall, cropping out in two tectonic windows, registered a different evolution, with a low- P history that evolved from low- to high- T under a high geothermal gradient. Several different P– T paths of the Mondoñedo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area. Following the initial crustal thickening, two main syn-convergence extensional shear zones developed. One of them occurs in the hangingwall, whereas the other affects the footwall unit. Both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet, and their activity is viewed as a consequence of the need for gravitational re-equilibration within the orogenic wedge. The most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low- P metamorphism is essentially a late phenomenon, and is linked to late-orogenic tectonic activity. In the Mondoñedo thrust sheet, our conclusions indicate that low- P metamorphism may also develop during convergence, and that this may occur in at least two cases. One is tectonic denudation of an allochthonous unit during its emplacement, and the other, thinning and extension at the footwall unit of an advancing thrust sheet. As a consequence, the low- P evolution may show different characteristics in different units of an orogenic nappe pile.

The Hercynian collision in the Armorican Massif : evidence of different lithospheric domains inferred from seismic tomography and anisotropy

Abstract The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma.The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North- and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GéoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes.The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian (≈350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere.The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.

Structural and Chronological Evidence for the India‐Eurasia Collision of the Early Paleocene in the Eastern Himalayan Syntaxis, Namjagbarwa

Abstract The eastern Himalayan syntaxis in Namjagbarwa is a high‐grade metamorphic terrain formed by the India‐Eurasia collision and northward indentation of the Indian continent into Asia. Right‐ and left‐lateral slip zones were formed by the indentation on the eastern and western boundaries of the syntaxis respectively. The Dongjug‐Mainling fault zone is the main shear zone on the western boundary. This fault zone is a left‐lateral slip belt with a large component of thrusting. The kinematics of the fault is consistent with the shortening within the syntaxis, and the slipping history along it represents the indenting process of the syntaxis. The Ar‐Ar chronological study shows that the age of the early deformation in the Dongjug‐Mainling fault zone ranges from 62 to 59 Ma. This evidences that the India‐Eurasia collision occurred in the early Paleocene in the eastern Himalayan syntaxis.

Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chlorite–mica local equilibria

The late greenschist deformation of the Cycladic Blueschists unit on Tinos Island evolves along a NE–SW gradient of non-coaxial strain across a major extensional shear zone. Deformation is more intense and the greenschist overprint is also more severe close to the shear zone that separates the Cycladic Blueschists from the upper ophiolitic unit in the NE of the island. The compositional variability of chlorite and K-white mica (KWM) in metapelites sampled along a NE–SW cross-section is characteristic of their P–T conditions of crystallization and is mainly characterized by variations of their Si, Al, Fe and Mg contents due to the Tschermak substitution (Si IV(Fe, Mg) VI=Al VIAl IV). Multi-equilibrium P–T estimates based on the assumption of local equilibrium indicate that each sample recorded a part of the P–T history, which depends on the distance to the contact to the main shear zone, i.e. the intensity and the pattern of deformation. The combination of P–T estimates obtained from all the samples collected along the transect provides a continuous P–T path which is composed of two parts. It provides insights on the syn- and post-orogenic exhumation for some of the Cycladic Blueschists. In particular, we could distinguish two stages in the exhumation process. A first stage in the blueschist facies occurred along a cold P–T path during the construction of the Hellenides. The second exhumation stage occurred along a warmer P–T path during the formation of the Aegean sea. Between the two stages of exhumation, there is a thermal reequilibration of the crust and a partial thermal overprint of the blueschist parageneses.

Thrust-related accretion of an Archaean greenstone belt in the Midlands of Zimbabwe

Detailed structural data from the Midlands greenstone belt show that, in contrast to pre-existing models, the evolution of the greenstone belt involved an early episode of thin-skinned thrusting affecting separate lithological domains. These display different sedimentary and structural-metamorphic histories prior to their juxtaposition across steep, west-directed thrust zones. The domains include 3.5Ga old gneiss, 2.67–2.88Ga, mafic–felsic volcanic units and early-syn-tectonic, clastic sedimentary sequences. Concomitant with thrusting along domain boundaries, upright folding and ‘minor’ shear accommodated strain within domains.The early thin-skinned, and later, steeper, thrusting events can be interpreted as progressive stages in an accretionary and crustal thickening sequence, possibly associated with under-plating or low-angle subduction. The clastic sedimentary sequences contain intraformational clasts and show coarsening upward cycles below major early thrust horizons, and may have formed on alluvial fans developing in front of west-moving nappes. Strike-slip motion on the main shear zones in the Midlands greenstone belt only occurred late in the tectonic history and there is no evidence that this resulted in large displacements. A shift of σ1, from E–W to N–S during the late stages of Archaean deformation of the greenstone belt can be attributed to extensional collapse following E–W shortening and crustal thickening of the Zimbabwe craton.

Alteration and mass transfer inferred from the Hirabayashi GSJ drill penetrating the Nojima Fault, Japan

Abstract Mineralogical and geochemical studies on the fault rocks from the Nojima–Hirabayashi borehole, south‐west Japan, are performed to clarify the alteration and mass transfer in the Nojima Fault Zone at shallow depths. A complete sequence from the hornblende–biotite granodiorite protolith to the fault core can be observed without serious disorganization by surface weathering. The parts deeper than 426.2 m are in the fault zone where rocks have suffered fault‐related deformation and alteration. Characteristic alteration minerals in the fault zone are smectite, zeolites (laumontite, stilbite), and carbonate minerals (calcite and siderite). It is inferred that laumontite veins formed at temperatures higher than approximately 100°C during the fault activity. A reverse component in the movement of the Nojima Fault influences the distribution of zeolites. Zeolite is the main sealing mineral in relatively deep parts, whereas carbonate is the main sealing mineral at shallower depths. Several shear zones are recognized in the fault zone. Intense alteration is localized in the gouge zones. Rock chemistry changes in a different manner between different shear zones in the fault zone. The main shear zone (MSZ), which corresponds to the core of the Nojima Fault, shows increased concentration of most elements except Si, Al, Na, and K. However, a lower shear zone (LSZ‐2), which is characterized by intense alteration rather than cataclastic deformation, shows a decreased concentration of most elements including Ti and Zr. A simple volume change analysis based on Ti and Zr immobility, commonly used to examine the changes in fault rock chemistry, cannot account fully for the different behaviors of Ti and Zr among the two gouge zones.

Hercynian tectono-thermal evolution associated with crustal extension and exhumation of the Lora del Río metamorphic core complex (Ossa-Morena zone, Iberian Massif, SW Spain)

The Lora del Rio metamorphic core complex corresponds to the lowermost, high-grade block below a Hercynian extensional shear zone. A peculiarity of this sector is that exhumation of the metamorphic core was the result of the activity of two low-angle, approximately perpendicular shear zones: the main and the secondary shear zones, both of which are separating three structural levels with distinct tectonometamorphic imprints. The Lora del Rio metamorphic core underwent rapid exhumation due to the combined action of both extensional shear zones. The Hueznar unit, which represents the median block, shows a complex evolution whereby the highest metamorphism occurs in relation to the secondary extensional structure, although most structures appear to be controlled by the main extensional shear zone. Metamorphism and deformation within the upper block (Los Miradores unit) are controlled by the underlying units. Recognition in the Ossa-Morena zone of extensional deformation processes (dated at 340 Ma), spatially and temporally related with the convergent deformations, can help in the establishment of comparisons and correlations with other sectors of the European Hercynian foldbelt.

Structural and chemical characterization of shear zones in the freshly activated Nojima fault, Awaji Island, southwest Japan

Behavior and role of each shear zone in a shallow fault zone of granitic origin during seismic cycles are revealed by comprehensive examinations of petrographic and chemical characterization on a fault zone in the Geological Survey of Japan (GSJ) drill core penetrating the Nojima fault which was activated during the 1995 Hyogoken‐Nanbu earthquake (M = 7.2). The GSJ core consists of granodiorite and porphyritic intrusive rocks including a Nojima fault zone which involves seven thin shear zones: main shear zone (MSZ, 625 m depth), upper cataclasite zone (UCZ), upper shear zone (USZ), lower shear zones (LSZ‐1 and LSZ‐2), and lower cataclasite zones (LCZ‐1 and LCZ‐2). These shear zones are generally surrounded by weakly pulverized and altered (fault‐related) rocks (WPAR) which generally show volume gain. The fault zone architecture is clarified as follows: (1) Total thickness of the Nojima fault zone is ∼70 m. (2) All shear zones except the older cataclasite zones (UCZ‐1, LCZ‐1, and LCZ‐2) were evolved from WPAR, indicating that pulverization and alteration of recent activity were more diffused at the initial stage of faulting and gradually localized to each shear zone. (3) The MSZ (2 m thick) can be regarded as a high‐velocity frictional zone with accompanying volume loss (compaction) and possibly with heat generation during coseismic periods. (4) The LSZ‐1 (3.6 m thick), located just beneath the MSZ and typically showing explosion brecciation texture, is also regarded as a coseismic shear zone. This zone could function as a trap zone for fluid or gas during postseismic/interseismic periods. (5) The LSZ‐2 (2.7 m thick), located around 710 m depth, contains foliated fault gouge enriched with clay minerals and characterized by a large degree of volume gain, possibly a result of slow velocity motion or creep during the interseismic and/or postseismic periods.

Tectonic facies and the archipelago-accretion process of the West Kunlun, China

A tectonic facies investigation carried out in the West Kunlun, China allows us to have worked out a tectonic model of orogen. The tectonic facies, from the north to the south, are composed of the following: 1. Southern Tarim tectonic realm; 2. North Kudi magmatic arc; 3. Kudi melange; 4. Kudi micro-continent; 5. main shear zone; 6. Xianan Bridge calc alkaline complex; 7. Mazar-Kangxiwar melange-accretion complex; and 8. Tianshuihai foreland fold-thrust belt. The tectonic facies 1->5 recorded the history of the northward subduction of the Prototethys and southward accretion of Eurasia in the Late Proterozoic-Early Paleozoic time, while the tectonic facies 6->8 recorded the history of the northward subduction of the Paleotethys and southward accretion of Eurasia in the Late Paleozoic-Early Mesozoic time, that of the tectonic evolution of the passive margin of the Qiangtang block, and that of the docking and the final amalgamation of the Qiangtang block to the Eurasian continent. The tectonic facies investigation has indicated that a complicated archipelago-accretion orogenesis took place in the West Kunlun orogen, which was the important character of southward growth of the Eurasian continent.

The metamorphic evolution of the high pressure mafic granulites of the Bacariza Formation (Cabo Ortegal Complex, Hercynian belt, NW Spain) ☆

The high pressure mafic granulites of the Bacariza Formation outcrop in the two uppermost structural units of the Cabo Ortegal Complex (La Capelada unit and Cedeira unit) were separated by a Variscan thrust. In both cases, they appear as heterogeneous metabasites in normal contact between ultramafic rocks and other more homogeneous and less differentiated metabasic rocks, also affected by catazonal metamorphism. The main difference between the mafic granulites in the two units is the degree of deformation, which is higher in the underlying Cedeira unit. Petrologie and mineralogical data indicate that the high-pressure (HP) granulites (Gt-Cpx ± Amp-Pl ± Qtz ± Scp-Rt ± Ilm-Czo) are already retrograde (M2 Stage), post-dating an earlier eclogite facies metamorphism (Ml Stage) characterised by the mineral associations: Gt-Cpx ± Amp ± Ky ± Qtz-Rt and Gt-Cpx ± Amp ± Qtz ± Zo-Rt. The main structure relatted to the exhumation processes is the development of a general mylonitic foliation that, although initiated in granulite facies conditions, was mainly equilibrated in amphibolite facies (M3 Stage). This foliation was affected by isoclinal folds, which led to the formation of the Variscan thrusts responsible for the present stacking position. Thrust conditions were transitional between amphibolite and greenschist facies (M4 Stage). Thermobarometric data point to different P—T exhumation paths in the two units. Estimated P—T conditions were higher in La Capelada unit during Ml (P ≥ 13 kbar; 860 °C) and M2 (15 kbar; 800 °C) than in the Cedeira unit (Ml: P ≥ 11 kbar, 770 °C; M2: 12 kbar; 750 °C). Temperatures for the M3 stage were comparable (720 °C) in both units but rocks from the Cedeira unit show a much bigger drop in pressure. This resulted in an isothermal decompression type path for the Cedeira unit, while both P and T decreased more steadily in La Capelada rocks. These were always located at deeper level than the Cedeira rocks before the Variscan stacking. The difference in the two paths is related to different exhumation rates; higher in rocks from the Cedeira unit than in those from La Capelada. Exhumation processes coeval with underthrusting, and a different location of the rocks with respect to the main shear zone responsible for the exhumation would account for the distinct paths.

Geochronological and kinematic constraints on crustal shortening and escape in a two-sided oblique-slip collisional and magmatic orogen, Paleoproterozoic Taltson magmatic zone, northeastern Alberta

The southern Taltson magmatic zone (south of 60°N) is a composite continental magmatic arc and collisional orogen resulting from the convergence of the Buffalo Head terrane with the Archean Churchill craton. Taltson basement (ca. 3.2–3.0 Ga and 2.4–2.14 Ga) and Rutledge River supracrustal gneisses (2.13–2.09 Ga) were intruded by voluminous I- and S-type magmatic rocks between 1.99 and 1.92 Ga. Taltson magmatic zone was deformed by three ductile shear zones: Leland Lakes, Charles Lake, and Andrew Lake, exhibiting both strike- and dip-lineated mylonitic domains. Kinematic data for shear zones are reported at microscopic, mesoscopic, and macroscopic (remotely sensed data) scale. We present field and U–Pb isotopic data (zircon and monazite) for magmatic and metamorphic rocks that constrain the timing of granulite to upper amphibolite-grade shearing in the Leland Lakes and Charles Lake (formerly Allan) shear zones to ca. 1938–1934 Ma. Foreland (easterly) vergent thrusting on the Andrew Lake shear zone is ca. 1932 Ma. Taltson shear zones were overprinted by widespread amphibolite- to greenschist-grade shearing, which is constrained by published 40Ar–39Ar and K–Ar dates on hornblende and muscovite to between ca. 1900 and 1800 Ma. We propose a crustal architecture, resembling a crustal-scale asymmetric flower structure, in which the Charles Lakes shear zone formed the fundamental shear zone of a middle to lower crustal sinistral transpression system that accommodated southward escape of crust in the upper plate of an oblique continental subduction–collision zone, with shortening partitioned into synchronous outwardly vergent thrust systems to the east and west of the main shear zone.

Structural evolution of the Paleoproterozoic Rio Itapicuru granite-greenstone belt (Bahia, Brazil): the role of synkinematic plutons in the regional tectonics

The Paleoproterozoic structural evolution of the Rio Itapicuru greenstone belt (RIGB, Bahia, Brazil) involves two tectonic events of different natures, both associated with magmatism. The first event (D1) developed in response to NW-SE shortening and was responsible for subhorizontal foliation, NW-SE trending lineation and SE-directed thrusts. D1 was coeval with the emplacement of rare 2130 Ma granodioritic plutons. The D2 tectonic event was characterized by left-lateral strike-slip tectonics, developed on steeply dipping D1 foliation and within numerous NS elongated ca 2080 Ma plutons. Quartz c-axis analysis within the granites shows that they experienced progressive left-lateral shearing from magmatic to solid-state conditions under decreasing temperatures. This fact, in addition to microstructural arguments, indicates syntectonic emplacement of these plutons. A zone of maximum deformation (the Main Shear Zone) has been identified in the centre of the RIGB on the basis of strain measurements. A tectonic model in which D1 thrusting event initiated basin closure before being replaced by D2 strike-slip motion is proposed. The Main Shear Zone corresponds successively to the main thrust zone and the main wrench fault during D1 and D2 events, respectively. Such a switch from orogen-normal thrusts to orogen-parallel transcurrent movements may represent a common progression during evolution of mountain belts in order to accommodate excess crustal thickness. However, because low-grade metamorphism and low intensity of the deformation demonstrate that excess crustal thickness was not attained, it is argued that granite emplacement and ascent played a significant role in triggering strike-slip tectonics in the RIGB.

An analysis of shear waves observed in VSP data from the superdeep well at Kola, Russia

SUMMARY Two VSPs have been analysed from the Kola superdeep borehole, located on the Kola Peninsula. The Kola borehole was drilled down to a depth of 12 260 m and VSPs were recorded in the interval 2150 to 6000 m. The VSPs sample the Proterozoic Pechenga complex, which consists of alternating metasedimentary and metavolcanic layers that range from greenschist to amphibolite-grade facies. The structural dip is 28-45' to the SSW, and a major shear zone (the Luchlompolsky fault) occurs at 4.5 km depth. The VSPs display strong transmitted- and reflected-mode converted energy from structural and lithologic boundaries. A kinematic (traveltime) ray-tracing modelling of the main compressional- (P-) and shear- (S-) wave events was performed to define the seismic boundaries. P- and S-wave velocities were estimated from the near-offset data, and V,/V, ratios were related to the lithology. A significant increase in the &/V, ratio is observed in the main shear zone at 4.5 km depth. Shear-wave splitting is identified by traveltime divergence (different apparent velocites) and orthogonal polarization of S phases in the far-offset VSP. The inferred polarization direction for the fast shear wave is N160W, clearly observed below 4400 m depth. Two models are suggested to explain the observed shear-wave splitting: intrinsic anisotropy caused by aligned hornblende minerals in the amphibolite-grade facies; and vertical cracks aligned N160W. The direction of crack alignment is not consistent with the present-day NW-SE maximum compressive stress field. However, it is consistent with the direction of the palaeostress, the direction of crack alignment at the surface and the fast direction obtained from analysis of shear waves in shallow VSPs. The velocity anisotropy is estimated to be 4-5 per cent with a local increase to 10 per cent in the Luchlompolsky fault zone.