Normal Shear Zones Research Articles

Late orogenic collapse and thermal doming in the northern Gondwana margin incorporated in the Variscan Chain: A case study from the Ozieri Metamorphic Complex, northern Sardinia, Italy

A branch of the South European Variscan chain is noticeably exposed in Sardinia. The early stage of collision between the Northern Gondwana margin and the Armorica Terrane Assemblage (ATA) generated syn-metamorphic folding and thrusting. The evidences of such deformation are well preserved in the nappe zone, a structural domain characterized by stacking of different tectonic units under metamorphism of Barrovian greenschist facies. A late, post-nappe, shortening, under retrograde metamorphic conditions, gave rise to wide, upright, N120–N160 trending antiforms that control the trend of the chain. The structural analysis of the Ozieri Metamorphic Complex (OMC) shows evidence of an important phase of late-Variscan extensional tectonics. Deformation results in, the formation of oppositely dipping, normal shear zones, which developed at upper and middle structural level along the limbs of major regional antiforms causing fabric reactivation, crustal thinning, and exhumation of the OMC core. Within the OMC, the activity of the shear zones was coeval with HT-LP metamorphism as suggests the occurrence of syn-kinematic cordierite + andalusite ± sillimanite + biotite. Whereas syntectonic dykes and a tonalite body in the deeper part of the OMC indicate that early emplacement of melt along shear zones and/or in the antiform hinges possibly supplied the heat for the anomalous thermal gradient and triggered the exhumation of a core complex-like structure.

Geologia e arcabouço estrutural do Complexo Lagoa Real, vale do Paramirim, Centro-Oeste da Bahia

The Lagoa Real Complex is constituted by 1.7 Ga granitoids that occur along the Paramirim valley. The granitoids are formed by the São Timóteo Granite and a set of alkali-gneisses that were crystallized during a compressional sin-metamorphic deformation episode. The carried out structural analysis has identified two deformation patterns. The compressional episode is represented by shear zones, compositional layering, milonite foliation and folds. The extensional episode is characterized by the nucleation of normal shear zones. The geometric patterns of the finite deformation are similar to the ones that occur in the Ituaçu synclinal where the Salitre e Bebedouro formations of Una Group outcrop. An ancient geometric pattern was not found in the area; consequently we propose that the deformation episodes were related to the neoproterozoic Orogeny.

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex, Central Eastern Desert, Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.

El Sibai gneissic complex, Central Eastern Desert, Egypt: Folded nappes and syn-kinematic gneissic granitoid sheets – not a core complex

The El Sibai area of the Central Eastern Desert (CED) of Egypt consists of an ophiolitic association of arc metavolcanics, ophiolitic rocks, mélange, metasediments and minor mafic intrusions; and a gneissic association of amphibolite, gneissic diorite, tonalite, granodiorite and granite. Previous studies of the El Sibai area have identified the gneissic association as a lower crustal infrastructure in sheared contact with upper crustal ophiolitic association suprastructure, and have presented it as an example of a metamorphic or magmatic core complex. Detailed structural remapping of the El Sibai area reveals that the gneissic association rocks are not infrastructural but form a unit within the ophiolitic association nappes. Furthermore, the El Sibai structure is not domal in shape, and is not antiformal. The main gneissic association rocks are tabular intrusions roughly concordant with the shears dividing the ophiolitic association into nappes, and are syn-kinematic with the nappe stacking event (∼700–650 Ma). The gneissic granite tabular intrusions and their ophiolitic host were later folded about upright NW–SE trending mainly open folds during a NE–SW directed shortening event (∼625–590 Ma). Subsequently, NW–SE regional extension effects became evident including low angle normal ductile shear zones and mylonites. The latest gneissic red granites are syn-kinematic with respect to these shear zones. Probably continuing from the low-angle shearing event were steep normal faults, and sinistral WNW and N–S trending transcurrent faults (∼590–570 Ma). The normal faults mark the southeastern and maybe also the northwestern limits of the El Sibai gneissic association rocks. The El Sibai complex is not a core complex, but exemplifies the overlap of NW–SE folding and NW–SE extensional which is a significant theme of CED regional structure.

Extension and gold mineralization in the Archean Kalgoorlie Terrane, Yilgarn Craton

This paper focuses on two extensional events affecting late Archean crust of the Kalgoorlie Terrane in the Eastern Goldfields, and which are associated with gold mineralization. These events bracket the duration of the crustal shortening event known as the Kalgoorlie Orogen. The early extension phase is preserved around the Raeside Batholith and gave rise to the earliest recognized structures, D 1e . This event is characterized by multi-directional horizontal extension, in contrast to unidirectional extension associated with core complexes. Extension uplifted the batholith in relation to the overlying greenstone sequence and produced recumbent folds. Gold, including the large deposits at Leonora, was deposited in normal shear zones around the batholith. We propose that extension triggered doming of gravitationally unstable granitic rocks buried under a greenschist sequence, and amplified the growth of extensional nappes. The late phase of extension is evident in a number of gold deposits in the Kalgoorlie Terrane. This phase is characterized by (a) overprinting crustal shortening structures that suggests it is the latest structure in all localities; (b) gently dipping foliation, associated with recumbent folds, which are dragged into moderately dipping, normal shear zones; (c) maximum stretching axis, NW–SE or N–S, at high angles to the maximum shortening axis inferred from regional shortening phases of the Kalgoorlie Orogen and (d) hosting gold. This phase records relaxation of the Kalgoorlie Orogen and the findings suggest that gold mineralization was not restricted to a single deformation phase but started before the Kalgoorlie Orogen and waned at the end of the orogeny.

Early exhumation of high‐pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey

Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high‐pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist‐facies Menderes nappes along its lower tectonic contact, the Cycladic‐Menderes thrust, effectively cutting out a ∼30‐ to 40‐km‐thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top‐to‐the‐NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼30–40 km. Detailed Rb‐Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust‐type and normal sense shearing can accomplish the early exhumation of deep‐seated rocks.

Collision tectonics involving juvenile crust: The example of the southern Finnish Svecofennides

This paper presents a reappraisal of the main structural and metamorphic features of the Southern Svecofennian Arc Complex (southern Finland) and provides new data that allow us to reconsider its tectonic evolution. Results emphasize that (1) regional shortening was accommodated by distributed thickening with no clear evidence for large-scale thrusting, (2) syn-convergence lateral flow of ductile crust most probably competed with crustal thickening since early stages of deformation, (3) no evidence exists for thickening-driven extensional collapse associated with normal shear zones, the latest structures developed being compressive, (4) exhumation appears basically driven by distributed erosion that competed with crustal thickening. Observations are interpreted in terms of a peculiar mechanical behaviour of a hot and weak juvenile lithosphere undergoing compression.

Formation of new continental crust in Western British Columbia during transpression and transtension

Crustal growth in the Coast Mountains, along the leading edge of the Canadian Cordillera, was the result of processes associated with horizontal flow of material during transpression and subsequent transtension, and the vertical accretion of mantle derived melts. From 85 to 58 Ma, as exotic terranes were translated northward during transpression, the crust was thickened to about 55 km, and melt that originated from a mix of mantle-derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones. When the orogen extended, between 58 and 50 Ma, there was large-scale decompression melting in the mantle and dehydration melting in the lower crust. Voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones. By 50 Ma, the final crustal thickness of the new continental crust was about 34 km. Comparison of seismic data with other crustal sections suggests that the crust-forming processes identified in western British Columbia have general applicability to models for the formation of continental crust.

The significance of gneissic rocks and synmagmatic extensional ductile shear zones of the Barud area for the tectonics of the North Eastern Desert, Egypt

The mainly tonalitic gneissic rocks, amphibolites, schists and rarer migmatites of the Barud area, at the southern margin of the Egyptian North Eastern Desert (NED) have previously been viewed as products of ultrametamorphism or granitization of pre-PanAfrican basement. The Qena–Safaga Line of approximately NW-striking steeply dipping faults was also regarded as marking the boundary between these NED rocks and the low grade metavolcanics and ophiolitic melange of the Central Eastern Desert (CED). Detailed investigation of the Barud area indicates that the amphibolites, schists and migmatites formed by shearing and medium grade metamorphism of similar arc metavolcanics to those of the CED in normal shear sense extensional ductile shear zones heated by numerous syn-kinematic dolerite, gabbro, diorite, granodiorite, tonalite and granite dykes. They are thus hot sheared equivalents of the CED metavolcanics and basic arc plutonites, accompanied by sheared mafic and felsic intrusive rocks, and are not deep-seated crystalline basement rocks. The shear zones are interpreted as having formed by arc-rifting, not necessarily reaching the stage of marginal basin formation. Arc-accretion structures and those produced by later orogen squeezing are also described. The somewhat gneissic Barud Tonalitic is found to be entirely magmatic. Following intrusion of the Barud Tonalite, and before or during Hammamat and Dokhan deposition, the NED experienced a rapid uplift relative to the CED (∼620–600 Ma) that was not achieved by thrusting along the Qena–Safaga Line.

Normal-sense shear zones in the core of the Higher Himalayan Crystallines (Bhutan Himalaya): evidence for extrusion?

Abstract Recent fieldwork in western Bhutan, dedicated to unravelling the tectonic structure of the mid-crustal rocks, indicates a complex deformation pattern in the Greater Himalayan Slab (GHS). A system of normal shear zones, striking NE-SW and steeply to moderately dipping to the SE, has been recognized within this extruding slab or wedge of crystalline rocks. The zones are characterized by well developed shear-sense indicators pointing to a top-down-to-SE sense of shear. The main Barrovian metamorphic minerals are bent and stretched by extensional shear bands and associated deformation mechanisms indicate a range of brittle-ductile deformation conditions. Normal shear zones are concentrated in the middle-upper part of the GHS and indicate a thrust-transport-parallel lengthening of the core itself. Vorticity analysis highlights a non-coaxial flow with pure and simple shear acting together during deformation (mean vorticity number bracketed between 0.63 and 0.76). These data, when compared to available data near the tectonic boundaries of the GHS, indicate an increasing component of pure shear towards the interior of the GHS. The ages of zircon overgrowths and monazites from a slightly deformed granite, 20.5 ± 0.5 Ma, and a mylonitic granite deformed into the shear zones, 17.0 ± 0.2 Ma, bracket the age of shear zone formation at close to 17 Ma. According to our data, the normal shear zones could well accommodate the pure shear component of deformation localized in the inner part of the extruding wedge/slab and is compatible with a channel flow model.

Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan‐Tibetan orogen

Plane strain, thermal‐mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens. In the models, radioactive self‐heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large‐scale horizontal flow. Channels can be exhumed and exposed by denudation focused on the high‐relief transition between plateau and foreland. We interpret the Himalaya to have evolved in this manner. Channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws, and well developed if there is an additional reduction in viscosity to 1019 Pa s. This reduction occurs from 700°C to 750°C in the models and is attributed to a small percentage of in situ partial melt (“melt weakening”). Model HT1 provides an internally consistent explanation for the tectonic evolution of many features of the Himalayan‐Tibetan orogenic system. Erosional exhumation exposes the migmatitic channel, equivalent to the Greater Himalayan Sequence (GHS), between coeval normal and thrust sense ductile shear zones, corresponding to the South Tibetan Detachment and the Main Central Thrust systems. Outward flow of unstable upper crust rotates these shears to low dip angles. In the model both the GHS and the Lesser Himalayan Sequence are derived from Indian crust, with the latter from much farther south. Similar models exhibit a range of tectonic styles, including the formation of domes resembling north Himalayan gneiss domes. Model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau.

Crustal channel flows: 2. Numerical models with implications for metamorphism in the Himalayan‐Tibetan orogen

Results from a thermal‐mechanical model (HT1) that includes midcrustal channel flow are compatible with many features of the Himalayan‐Tibetan system. Radioactive self‐heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot, low‐viscosity midcrustal channel and a broad plateau. Channel material, corresponding to the Greater Himalayan Sequence (GHS), flows outward from beneath the plateau in response to topographically induced differential pressure. At the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler, newly accreted material corresponding to the Lesser Himalayan Sequence (LHS). The model channel is bounded by coeval thrust and normal sense ductile shear zones, interpreted to represent the Main Central Thrust (MCT) zone and South Tibetan Detachment system, respectively. Inverted metamorphism associated with the model MCT zone results from distributed ductile shear along the MCT and extrusion of the hot channel. A variety of model P‐T‐t path styles, resembling those observed in the GHS and LHS, are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model. Predicted times of peak metamorphism, cooling, and erosion of metamorphic facies are generally compatible with observations, although model GHS cooling ages are too young. The times of M1 and M2 metamorphic “events” observed in the GHS correspond to model times of maximum burial and maximum heating, respectively. The results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens.

Seismic image of the deep crust at the eastern margin of the Alps (Austria): indications for crustal extension in a convergent orogen

A 39-km-long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the Eastern Alps (Austria). The ESE–WNW-trending, low-fold seismic line crosses Austroalpine basement units and extends approximately from 20 km west of the Penninic window group of Rechnitz to 60 km SSE of the Alpine thrust front. The explosive-source seismic data reveals a transparent shallow crust down to 5 km depth, a complexly reflective upper crust and a highly reflective lowermost crust. The upper crust is dominated by three prominent west-dipping packages of high-amplitude subparallel reflections. The upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low-angle normal shear zones related to the Miocene exhumation of the Rechnitz metamorphic core complex. In the western portion of the upper crust, east-dipping and less significant reflections prevail. The lowermost package of these reflections is suggested to represent the overall top of the European crystalline basement. Along the western portion of the line, the lower crust is characterised by a 6–8-km-thick band of high-amplitude reflection lamellae, typically observed in extensional provinces. The Moho can be clearly defined at the base of this band, at approximately 32.5 km depth. Due to insufficient signal penetration, outstanding reflections are missing in the central and eastern portion of the lower crust. We speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower Moho in the east, preferable at about 30.5 km depth. The high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust, the latter interpreted to represent broad extensional mylonite zones, emphasises the latest extensional processes in accordance with eastward extrusion.

Tectonometamorphic Evolution of the Austroalpine Nappe Complex in the Central Eastern Alps—Consequences for the Eo-Alpine Evolution of the Eastern Alps

In the east-central part of the Eastern Alps, three major deformation events can be distinguished within the Koralm Complex and adjacent units (Plankogel Complex, Gleinalm Complex, Seckau Crystalline Complex, Paleozoic of Graz). A first deformation event D1 is characterized by the formation of a penetrative foliation and an E-W stretching lineation. Remnants of deformational micro-structures indicate a top-to-the-west sense of shear during this deformation event. Most of the D1-related fabrics were overprinted by subsequent metamorphism. This metamorphic event did affect the presumed tectonic boundary between the Koralm Complex and the Gleinalm Complex below. Particularly, D2 is related to the Plattengneis shear zone, which formed in the uppermost structural sections of the Koralm Complex, characterized by a N-S-oriented stretching lineation. Eclogites in the footwall have been affected by this deformation event, too. This deformation event is associated with pure shear in the central parts of the Koralm Complex, probably with top-to-the-south displacement in the southern parts, and top-to-the-north displacement in the northern parts. Deformation within the Plattengneis and the eclogites below occurred along the decompressional path, indicated by decreasing minimum pressures within the eclogites, and by northward and southward decreasing pressures and temperatures. The Plattengneis shear zone continuously passes into a low-angle normal fault in the northeastern part of the Koralm, forming the contact between the Koralm Complex and the Paleozoic of Graz. Thus, the Plattengneis shear zone primarily formed as an extensional structure and triggered exhumation of the eclogites. D3-related structures are restricted to distinct low-angle normal shear zones along the northern and southern margins of the Koralm Complex, with top-to-the-N/NE and top-to-the-S/SE displacement, respectively. These are related to the juxtaposition of exhumed high-pressure rocks of the Koralm Complex, and medium- to low-grade metamorphic units above. According to this evolution, the Cretaceous collisional process (Eo-Alpine cycle), which formed the present Austroalpine Nappe Complex, may be subdivided into two distinct phases: The first phase is the (ES)E-ward subduction and closure of the Hallstatt-Meliata Basin, resulting in the assembly of the Upper Austroalpine Nappe Complex. After closing of the Hallstatt-Meliata oceanic basin during the Late Jurassic, the Cretaceous orogeny in the Eastern Alps encompasses the collision between (south)easternmost parts of the Austroalpine continental crust and a continental fragment to the east. The second phase involves the southward underplating of the southern Apulian continental margin and resulted in the imbrication of the Middle Austroalpine basement complex. These units were additionally affected by pronounced metamorphim, increasing from greenschist-facies conditions in the northern parts to amphibolite- and eclogite-facies conditions in its southern-most parts. Continuous underplating was accompanied by extension in the internal parts of the orogen, resulting in the formation of an extensional detachment in the lower crust (Plattengneis shear zone), and exhumation of high-pressure metamorphic rocks during the Late Cretaceous. Coeval extension in the upper plate resulted in the formation of the Gosau sedimentary basins. Toward the north, the Plattengneis shear zone continuously climbed toward shallower crustal levels, and passed into a foreland-directed thrust. This thrust is conjectured to have affected the Upper Austroalpine Nappe Complex as well as the formation of distinct out-of-sequence thrusts.

Mazatan metamorphic core complex (Sonora, Mexico): structures along the detachment fault and its exhumation evolution

The Mazatán Sierra is the southernmost metamorphic core complex (MCC) of the Tertiary extensional belt of the western Cordillera. Its structural and lithological features are similar to those found in other MCC in Sonora and Arizona. The lower plate is composed of Proterozoic igneous and metamorphic rocks intruded by Tertiary plutons, both of which are overprinted by mylonitic foliation and N70°E-trending stretching lineation. Ductile and brittle–ductile deformations were produced by Tertiary extension along a normal shear zone or detachment fault. Shear sense is consistent across the Sierra and indicates a top to the WSW motion. The lithology and fabric reflect variations in temperature and pressure conditions during extensional deformation. The upper plate consists mainly of Cambrian–Mississippian limestone and minor quartzite, covered by upper Cretaceous volcanic rocks, and then by Tertiary syntectonic sedimentary deposits with interbedded volcanic flows. Doming caused uplift and denudation of the detachment, as well as successive low-angle and high-angle normal faulting across the western slope of Mazatán Sierra. An 18±3 Ma apatite fission-track age was obtained for a sample of Proterozoic monzogranite from the lower plate. The mean fission-track length indicates rapid cooling and consequent rapid uplift of this sample during the last stage of crustal extension.

10 km Minimum throw along the West Bohemian shear zone: Evidence for dramatic crustal thickening and high topography in the Bohemian Massif (European Variscides)

The West Bohemian shear zone (WBSZ) forms a steep collapse structure along which east-side- down normal movements led to the juxtaposition of the relatively cold Cadomian basement of the Tepla-Bar- randian unit against high grade Moldanubian rocks. Syn- kinematic plutons straddle the WBSZ. The Mutenin plu- ton intruded into Moldanubian crust at a depth of 23±4 km as derived by using Al-in-hornblende barome- try. The Tepla-Barrandian Babylon pluton intruded at <12 km depth as indicated by phengite barometry and petrogenetic considerations. Both emplacement depths, together with mineral cooling ages, result in a minimum vertical displacement of 10 km between 340 and 320 Ma. This large throw could be explained by over- thickened crust that was weakened from below. The al- kaline signature of the Mutenin diorite indicates that mantle melting was important to thermally weaken the crust at 340 Ma. The cold Tepla-Barrandian upper crust sank into its weak, partly molten Moldanubian substra- tum, resulting in elevator-style movements, not only along the WBSZ, but also along the Hoher Bogen and Central Bohemian shear zone. All these ductile normal shear zones were active simultaneously during the Low- er Carboniferous and dip steeply towards the Tepla-Bar- randian unit that probably formed a highly elevated pla- teau at this time.

Study of the radioactive disequilibrium of the volcanic rocks from the Trindade Island (Brazil)

perature allochtonous (Socorro-Guaxupe nappe). The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain with extensional structures in normal shear zones and compressive structures exhibiting E-NE transport. Tourmaline leucogranites in stratiform bodies and diatexitic gneisses occur frequently. There is a metamorphic transition, north to south, from the kyanite zone (and metabasic rocks in eclogite facies) to kyanite and sillimanite coexistence, until kyanite broken and exclusive sillimanite presence. The S2 texture foliation on mineral assemblage, suggests a decompressive path, where metabasics boudins preserve paragenese eclogite facies (Omp-Pl-Grt-Rt-Qtz) with simplectite textures between Cpx-Pl-Qtz. Externally, corroded garnets wrapped by plagioclase coronas, in amphibole matrix, represent the amphibolite facies reequilibration (MgHbl-Pl-Grt-Zo-Ilm-Ttn). Metapelites (RtKy-Grt-Ms-Bt-Pl-Qtz) may show Sil-Pl-Qtz coronitic intergrowths around garnets. The maximum temperatures for the distinguished metamorphic domains (14 samples), vary from 710 ± 30◦C (Ky domain) to 730 ± 50◦C (Sil domain). Although the pression vary at 17 ± 0.5 to 6 ± 0.5kbar. P-T conditions at 720 ± 50◦C and 11.5 ± 2.5kbar marks the kyanite assemblage re-equilibration (amphibole facies). P-T conditions 625 ± 55◦C and 6.8 ± 2kbar represent the cooling path (Ky-Sil fields). The high pression decompressive path suggests an extrusional process, immediately after buried at about 60km deep. Fast exhumation, process controlled by convergent events, derived by the Sao Francisco plate subductions and tectonic erosion take these units, isothermally, to upper levels (20-33 km). Later, the metamorphic path shift toward a near-isobaric cooling though in a slower extrusional process. P-T path continuity through three metamorphic domains suggests a sin-metamorphic process (S2 foliation) as a continuous effect S1 × S2, without diacronism or ensuing event superposition. However, large amount of leucogranites denotes a regional thermal event, subsequent and superimposed, responsible for quartz static polygonization in schists/gneisses and for Ms breakdown with microcline crystallization (730oC/6kbar) in the vicinity of large granite plutons. — ( December 14, 2001 ) . ∗ Supported by FAPESP 00/11389-6 and 98/15624-8. STUDY OF THE RADIOACTIVE DISEQUILIBRIUM OF THE VOLCANIC ROCKS FROM THE TRINDADE ISLAND (BRAZIL) Rosana N. Santos and Leila S. Marques Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de Sao Paulo, 01065-970, Sao Paulo, SP. Departamento de Fisica, Pontificia Universidade Catolica de Sao Paulo, 01303-050, Sao Paulo, SP. Presented by Antonio C. Rocha-Campos

A late Neoproterozoic magmatic core complex in the Eastern Desert of Egypt: emplacement of granitoids in a wrench-tectonic setting

Core complexes within the Eastern Desert of Egypt evolved in the course of orogen-parallel extension during Neoproterozoic plate convergence. In the Sibai Core complex, more than 90% of the exposed surface consists of magmatic rocks. In this study four magmatic suites have been distinguished. They are geochemically characterised and their intrusion ages are determined using the Pb/Pb single zircon evaporation technique. Results show that all magmatic suites formed during Neoproterozoic tectonics. The oldest, calc-alkaline rocks of group (I) gave ages of 670–690 Ma and are related to subduction processes in an island-arc setting. Subsequent orogen-parallel extension was accompanied by the intrusion of group (II) and (III) granitoids yielding ages of 655 and 645 Ma, respectively. The younger, felsic granites of group (IV) intruded along pre-existing structures and exhibit variable composition. We relate emplacement of various granitoids within the Sibai to progressively evolving structures. Group (I) represents the deep level of an island arc and exhibits nappe stacking structures related to oblique convergence in the late Neoproterozoic. Group (II) granitoids have been emplaced at mid-crustal levels and portray transpression and orogen-parallel extrusion dynamics. Related structures are represented by an older set of sinistral shear zones. Granitoids of group (III) intruded upper crustal levels during exhumation of the Sibai Core complex. Their emplacement was controlled by the pull-apart regime of a younger set of sinistral shear zones and associated normal shear zones. Denudation due to ongoing orogen-parallel extension is evident from deposition of sediments within the adjacent intramontane Kareim molasse basin. The sediments were intruded by post-tectonic granitoids of group (IV). Close relations between core complex formation and ongoing magmatic activity define the Sibai as a magmatic core complex.

Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western Mediterranean

The Ligurian basin, western Mediterranean Sea, has opened from late Oligocene to early Miocene times, behind the Apulian subduction zone and partly within the western Alpine belt. We analyze the deep structures of the basin and its conjugate margins in order to describe the tectonic styles of opening and to investigate the possible contributions of forces responsible for the basin formation, especially the pulling force induced by the retreating subduction hinge and the gravitational body force from the Alpine wedge. To undertake this analysis, we combine new multichannel seismic reflection data (Malis cruise, 1995) with other geophysical data (previous multichannel and monochannel seismic sections, magnetic anomalies) and constrain them by geological sampling from two recent cruises (dredges from Marco cruise, 1995, and submersible dives from Cylice cruise, 1997). From an analysis of basement morphology and seismic facies, we refine the extent of the different domains in the Ligurian Sea: (1) the continental thinned margins, with strong changes in width and structure along strike and on both sides of the ocean; (2) the transitional domain to the basin; and (3) a narrow, atypical oceanic domain. Margin structures are characterized by few tilted blocks along the narrow margins, where inherited structures seem to control synrift sedimentation and margin segmentation. On the NW Corsican margin, extension is distributed over more than 120 km, including offshore Alpine Corsica, and several oceanward faults sole on a relatively flat reflector. We interpret them as previous Alpine thrusts reactivated during rifting as normal faults soling on a normal ductile shear zone. Using correlations between magnetic data, seismic facies, and sampling, we propose a new map of the distribution of magmatism. The oceanic domain depicts narrow, isolated magnetic anomalies and is interpreted as tholeitic volcanics settled within an unroofed upper mantle, whereas calcalkaline volcanism appears to be discontinuous but massive and has jumped in space and time, from the beginning of rifting on the Ligurian margin (∼30 Ma), toward the Corsican margin at the end of the Corsica‐Sardinia block rotation (∼16 Ma). This space and time shift reveals the importance of the rollback of the Apulian slab and of the migration of the Alpine‐Apennines belt front toward the E‐SE for driving basin formation. We also state that initial rheological conditions and inherited crustal fabric induce important changes in the styles of deformation observed along margins and between conjugate margins. In the NE Ligurian basin the prerift Alpine crustal thickening together with slow rollback velocity likely contribute to distribute strain across the whole NW Corsican margin, whereas farther south the inherited Hercynian structural pattern combined with a faster rollback of the subducting plate tend to focus the extension at the foot of the margin, up to the Sardinian rift which ends within the SW Corsican margin. Therefore the mode of opening and the margin structures mainly depend on the balance between intrinsic, inherited crustal heterogeneity (fabric and rheological changes) and external conditions imposed by rollback of the subducting lithosphere.

Mechanisms for folding of high-grade rocks in extensional tectonic settings

This review of structures developed in extensional high-grade terrains, combined with results of centrifuge analogue modelling, illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust-thickened orogen. Several extensional fold mechanisms (such as folding within detachment shear zones) are similar to those in contractional settings. The metamorphic P– T– t path, and not fold style or mode of formation, is therefore required to determine the tectonic setting in which some folds developed. Other mechanisms such as rollover above and folding between listric normal shear zones, and folding due to isostatic adjustments during crustal thinning, are unique to extensional tectonic settings. Several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation. It is shown that isoclinal recumbent folds refolded by open, upright folds may develop during regional extension in the deep crust. Folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment, or from enhanced back-rotation of layers between normal shear zones. During back-rotation folding, layers rotated into the shortening field undergo further buckle folding, and all may rotate towards orthogonality to the maximum shortening direction. This mechanism explains the presence of many transposed folds, folds with axial planar pegmatites and folds with opposite vergence in extensional terrains. Examples of folds in high-grade rocks interpreted as forming during regional extension included in this paper are from the Grenville Province of Canada, Norwegian Caledonides, Albany Mobile Belt and Leeuwin Complex of Western Australia, Ruby Mountains in the Basin and Range Province of Nevada, the Atâ Sund area of Greenland, the Napier Complex of Enderby Land in East Antarctica and the Kigluaik Mountains in western Alaska.