Extensional Shear Zones Research Articles

Thrusting and Extension in the Southern Dora‐Maira Ultra‐High‐Pressure Massif (Western Alps): View from Below the Coesite‐Bearing Unit

The Dora-Maira ultra-high-pressure (UHP) unit, exposed in the internal Western Alps, is a ∼1-km-thick continental slice sandwiched between lower-pressure rock units. The contact at the base of the UHP unit is a major contractional shear zone that thrust the UHP unit and the overlying continental and oceanic units onto the Sanfront-Pinerolo unit. The latter, metamorphosed under blueschist-greenschist conditions, is the lowest structural unit exposed in the Western Alps. Our analysis indicates that all rock units of the Dora-Maira massif were sheared and juxtaposed together within a top-to-the-SW, large-scale, medium-temperature/low-pressure ductile shear zone in the middle crust. Thrust faulting was accompanied and followed by vertical thinning that progressively reduced the thickness of the orogenic wedge and brought the ultra-high-pressure (UHP) unit into the brittle crust. Contractional tectonic contacts in the southern Dora-Maira were concealed by late extensional structures. The majority of the extensional structures postdated the emplacement of the UHP unit over the Sanfront-Pinerolo unit, implying that extension lagged behind much of the exhumation of the UHP unit. Extensional shear zones in the Dora-Maira and elsewhere in the Western Alps are parallel to the strike of the orogen; their overall effect was to widen the orogenic belt in the (north)east-(south)west direction. The sense of extensional movement was toward the frontal part of the orogen so that rather than defining tectonic wedges formed during extrusion, the extensional structures represent lateral spreading of the mid- to upper crust of the internal Western Alps toward the foreland. Rapid compression of the Dora-Maira was coeval with the transition from marine flysch deposits to molasse sedimentation in the foreland of the Western Alps. The switchover from burial to exhumation and the lateral spreading of the upper crust in the internal Western Alps were coupled by readjustment of the entire orogenic wedge.

Extensional collapse of the late Neoproterozoic-early Palaeozoic East African-Antarctic Orogen in central Dronning Maud Land, East Antarctica

Abstract The East African-Antarctica Orogen resulted from the continent-continent collision of East and West Gondwana, or parts thereof, during the Pan-African event at c . 650–510 Ma. The collision overprinted large areas of older, mainly Mesoproterozoic, crust up to granulite facies grade in East Antarctica. The collision history is well documented by folding and thrusting, isothermal decompression and metamorphic zircon growth at c. 580–560 Ma (Pan-African I). The convergence was succeeded by an extensional phase, probably representing orogenic collapse. This Pan-African II event at c . 530–510 Ma is characterized by large-scale extensional structures, finally resulting in the post-tectonic intrusion of voluminous A2-type granitoids. In central Dronning Maud Land the Pan-African II event started with the intrusion of syntectonic igneous rocks within an overall extensional setting. Two new SHRIMP data from gabbro zircons of the Zwiesel Gabbro give ages of 521±5.6 and 527±5.1 Ma. These ages are interpreted as crystallization ages and confirm the interpretation that the gabbro was emplaced early during the Pan-African II event. The gabbro was intruded by a network of leucogranite dykes and veins. Whereas the gabbro appears entirely undeformed, the leucogranite dykes are strongly mylonitized along extensional shear zones, indicating pronounced strain partitioning of the gabbro complex. Within the leucogranite mylonites, large tension gashes developed during mylonitization, indicating very high strain rates. Quartz c-axis orientations from quartz of the tension gashes show a distinct cross-girdle that formed during pure shear deformation. Fluid inclusion data from the leucogranite mylonites and the associated tension gashes mainly reveal recrystallization-related intracrystalline CO 2 -dominant inclusions with relatively low densities of < 1 g cm −3 . The fluid inclusion data are interpreted to represent the last stages of a retrograde P-T path that is characterized by simultaneous cooling and decompression during extensional exhumation, probably succeeding the collapse of overthickened crust. A comparable orogenic collapse of the East African-Antarctic Orogen is reported from other parts of the orogen, such as from western Madagascar and the northern Arabian-Nubian Shield.

Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

We describe two shear zones in the Otago Schist of the Torlesse accretionary wedge, South Island, New Zealand: the north‐dipping Rise‐and‐Shine Shear Zone (RSSZ) and the south‐dipping Cromwell Gorge Shear Zone (CGSZ). Kinematic indicators (shear bands and asymmetric folds) indicate top‐north movement for the RSSZ and top‐south transport for the CGSZ. Back rotation of the shear zones into their Late Cretaceous orientation and consideration of the relationship of the shear zones to arching of the Otago Schist show that both shear zones are extensional. Offset of textural zones suggests up to 15 km of dip‐slip displacement on the RSSZ and probably a similar amount of slip for the CGSZ. We speculate that the gold‐mineralised RSSZ may be the western continuation of the late Mesozoic gold‐bearing Hyde‐Macraes Shear Zone in eastern Otago, forming a c. 100 km long extensional shear zone on the northern flank of the Otago Schist. Age constraints suggest that shear zone formation took place between 135 and 105 Ma. The shear zones aided the final exhumation of the deeper parts of the Otago Schist. We discuss whether normal shearing is related to syn‐orogenic supercritical tapering of the Torlesse wedge, or due to post‐orogenic New Zealand‐wide Albian rifting.

Is there any detachment in the Lower Dolpo (western Nepal)?

A structural transect in the Lower Dolpo highlights that the deformation and metamorphism of the Tibetan Zone (TZ) increase toward the bottom of the sequence. The contact with the underlying HHC is marked by a metamorphic jump from amphibolite facies in the carbonatic rocks of the upper part of the HHC to greenschist facies marbles in the TZ. Moreover, the HHC and the TZ show different metamorphic histories. The contact zone shows a strain increase accompanied by asymmetric folds with a top-to-the-northeast vergence, connected to a down-to-the-northeast tectonic transport. The contact is interpreted as an extensional shear zone, connected to the South Tibetan Detachment System. To cite this article: R. Carosi et al., C. R. Geoscience 334 (2002) 933–940.

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.

Late Paleozoic polyphased tectonics in the SW Belledonne massif (external crystalline massifs, French Alps)

In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian-Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Rioupéroux-Livet plutono-volcanic units. Micaschist slices have also been inserted within the Rioupéroux-Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7–8 kbar, 600–650 °C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Rioupéroux-Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest-southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

A two-stage evolution of the Neoproterozoic Rayner Structural Episode: new U–Pb sensitive high resolution ion microprobe constraints from the Oygarden Group, Kemp Land, East Antarctica

U–Pb sensitive high resolution ion microprobe (SHRIMP) zircon analyses have been used to place time constraints on the Proterozoic reworking of Archaean rocks during the Rayner Structural Episode in the Oygarden Group of islands, Kemp Land, East Antarctica. Felsic orthogneiss that preserves evidence for foliations that pre-date the Rayner Structural Episode, was deformed by two events during the Rayner Structural Episode: the first characterised by east-directed sub-horizontal thrusting, and the second by a regional scale south-dipping extensional shear zone. Metamorphic overgrowths on zircon grains from felsic orthogneiss, which have 929±12 and 924±17 Ma ages, are interpreted to have formed during granulite facies metamorphism that accompanied the thrusting event. Crystallisation of zircon in migmatite that also formed during this event, occurred in two stages: 904±16 and 884±24 Ma. Overgrowths on zircon in a pegmatite that intruded during the second event formed at 931±14 Ma. Charnockite that was emplaced in the Stillwell Hills prior to a regional thrusting event, was metamorphosed at 913±8 Ma. However, the distribution of isotopic data suggest the charnockite may have intruded at ca. 960 Ma. The ages for regional thrusting in the Oygarden Group and the Stillwell Hills correlate, suggesting that the Rayner Structural Episode did not affect Kemp Land until ca. 930 Ma. This age for peak metamorphism in Kemp Land is therefore 50 Myr younger than the age inferred for the Rayner Structural Episode further to the east on the Mawson Coast and south in the northern Prince Charles Mountains. The ages from the Oygarden rocks correlate with the second half of the orogenic episode seen on the Mawson Coast and northern Prince Charles Mountains, consistent with the possibility of two phases of collision in a transition from an Andean-style margin to continent–continent collision between 1000 and 900 Ma. Previously unreported ca. 1600–1650 Ma ages were also discovered in orthogneiss and pegmatite samples, indicating a thermal event in Kemp Land at this time. The 1600–1650 and 930–900 Ma ages from the Oygarden Group correlate with metamorphic and magmatic ages from the Eastern Ghats, placing further constraints on the correlation between these metamorphic belts.

Tertiary S-C mylonites from the Bajánsenye-B-M-I borehole, western Hungary

Ultramylonites and margarite-bearing quartz-feldspar S-C mylonites, containing amphibolite lenses with symplectitic texture, were encountered in a borehole (Bajánsenye-B-M-I) close to the west of the Transdanubian Central Range Unit. These rocks demonstrate a ductile, horizontal extensional shear zone attaining a thickness of 300 m. Microstructural data, mineral parageneses and mineral chemistry of these rocks indicate a multistage metamorphic evolution, which is consistent with that of the Koralm-Pohorje basement. The youngest mylonitic event (Early Tertiary) took place in the Bajánsenye mylonites at 430-450oC (greenschist-facies); it rejuvenated coarse-grained muscovite crystals of eo-Alpine age (Early-Middle Cretaceous). The radiometric data presented in this paper demonstrate for the first time an important Early Tertiary tectonic zone in this area.

Late Paleozoic polyphased tectonics in the SW Belledonne massif (external crystalline massifs, French Alps)

In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian–Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Rioupéroux–Livet plutono-volcanic units. Micaschist slices have also been inserted within the Rioupéroux–Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7–8 kbar, 600–650 °C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Rioupéroux–Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest–southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt.

Structural and metamorphic constraints on ca. 70 Ma deformation of the northern Valhalla complex, British Columbia: implications for the tectonic evolution of the southern Omineca belt

Penetrative deformation occurred ca. 70 Ma ago throughout the northern Valhalla complex in Valhalla and Passmore domes and in the Gwillim Creek shear zone, exposed at the deepest structural levels in both domes. Intense strain ( S T) in the Gwillim Creek shear zone (domain II) was synchronous with and outlasted deformation ( D 2) throughout the northern complex (domain I). Upper-amphibolite facies peak mineral assemblages define the predominant foliation. Temperature and pressure results, determined from microdomains with established relationships to reaction textures and microstructures, provide constraints on conditions under which deformation occurred. Deformation was synchronous with and outlasted peak metamorphic conditions at all structural levels. Peak conditions of 825°C and 730 MPa and 850°C and 840 MPa were determined for domains I and II, respectively. This was followed by cooling and retrograde garnet breakdown at conditions of 715°C and 490 MPa and 765°C and 730 MPa in domains I and II, respectively. The faster cooling rate per kilometer of exhumation for domain II relative to domain I is consistent with a model of conductive cooling via thrusting of domain II on to a cold footwall. Metamorphism is interpreted to have resulted from crustal thickening and burial to depths of ca. 25 km based on an inferred clockwise P– T path and the paucity of Late Cretaceous intrusions. Lack of retrograde metamorphism throughout the complex and the high degree of annealing of microstructures indicates that the rocks remained above greenschist-facies conditions until they were exhumed in the Early Tertiary on the Valkyr–Slocan Lake extensional shear zone system. Previous workers have determined that the peak of metamorphism occurred at 72–67 Ma in a restricted locality in the core of Passmore dome, near Vallican. Our study links this dated metamorphism with the structural evolution and metamorphic history throughout the area, and shows that supracrustal rocks at all structural levels in Valhalla and Passmore domes underwent the same metamorphic and deformation event as those near Vallican. Therefore, we assign a ca. 70 Ma age to the penetrative, high-temperature deformation in northern Valhalla complex and the Gwillim Creek shear zone. This coincides with a major period of shortening in the Rocky Mountains of the Foreland belt. Strain in northern Valhalla complex may represent a local transient shear zone that accommodated crustal thickening in the hinterland during orogen-scale compression, or it may be an exhumed part of the basal detachment of the Rocky Mountains.

Strain localization by fracturing and reaction weakening — a mechanism for initiating exhumation of subcontinental mantle beneath rifted margins

Abstract Rift-related strain localization in spinel lherzolite from an exhumed passive continental margin in the Southern Alps involved two stages. (1) Critical fracturing coincided with heterogeneous nucleation of plagioclase, olivine, and hornblende aggregates to form discrete, ultrafine-grained (0.5–0.6 µm) shear zones oriented at high angles to the pre-existing foliation in the host rock. The syntectonic replacement of spinal lherzolite by lower pressure, plagioclase-hornblende lherzolite documents extensional exhumation under high temperature (700–900 °C) conditions accompanied by limited fluid infiltration. Deformation involved a combination of dislocation creep (ol) and diffusion-accommodated viscous granular flow (plag, ol, hbl aggregates). (2) Hydrous deformation at lower temperatures (200–400 °C) involved the formation of serpentine-chlorite mylonite and cataclasite along discrete, anastomozing shear zones oriented at low angles to the pre-existing foliation. Both stages involved drastic weakening, particularly once the shear zones coalesced subparallel to the extensional shearing plane. The top of the lithospheric mantle was initially strong, but is inferred to have become weaker than both the underlying mantle and the overlying mafic lower crust. The interconnection of such strong-then-weak delamination zones to form trans-lithospheric extensional shear zones accelerated rifting and led to the exhumation of subcontinental mantle during the late stages of continental breakup.

Melt segregation structures within the Inzie Head gneisses of the northeastern Dalradian

Synopsis The Inzie Head gneisses of the NE Dalradian are syntectonic metapelitic schollen migmatites containing numerous sill-like bodies of granite and diorite. Leucosomes with clear evidence of melt-present crystallization exhibit a wide variety of segregation structures. Initial in situ melting occurred within, or at the margins of, fertile metasedimentary horizons and was strongly controlled by variations in bulk-composition. Intergranular melt-pockets coalesced into thin (<10 mm), discontinuous stromatic horizons which subsequently joined laterally and/or became interconnected in extensional shear-zones. A diktyonitic structure formed, enabling buoyancy aided upward migration of melt into low-strain sites such as boudin-necks and shear-zones. Metre-scale leucosome channels containing abundant schollen and having diffuse, unchilled contacts against the migmatites were supplied from this network and may represent the foci of H 2 O-rich fluid influx. Bodies of nebulitic granite contain abundant ‘ghost’ schollen in the final stages of assimilation. Diorite sills show extensive magma mingling and hybridization with granitic leucosome and were clearly contemporaneous with the crustal melting. A variably developed flattening fabric within the migmatites was in part due to inflation following intrusion of the diorites. Immediately beneath diorite intrusions, melt loss from the migmatites was pronounced indicating that pure-shear dominated deformation is a strong driving force for melt segregation. Garnetiferous aplites at higher crustal levels and contemporaneous with the peak of regional metamorphism represent melt frozen in channelways that potentially contributed to nearby, large-scale Grampian granitic bodies.

Styles of footwall uplift along the Simplon and Brenner normal fault systems, central and Eastern Alps

The Simplon and Brenner extensional shear zones of the central and Eastern Alps, respectively, are low‐angle detachments that accommodated orogen‐parallel extension and unroofed midcrustal rocks in their footwalls. An integrated field and fluid inclusion study of the Simplon low‐angle shear zone reveals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the Simplon footwall, including synthetic (west down) and antithetic (east down) semibrittle and brittle structures, and strike‐slip, extensional, and contractional structures, all of which are consistent with southwest directed extension along the main detachment zone. Fluid inclusion analysis of kinematically referenced inclusion arrays indicates that west down, semibrittle, and brittle shears were active at conditions above 300°–450°C and 14–26 km. Antithetic deformation along brittle faults occurred at much shallower conditions of <300°C and 3–7 km. Deformation along these structures accommodated footwall uplift by a subvertical simple shear mechanism in the southern part of the Simplon region. These results are strikingly similar to those from the Brenner shear zone. In contrast with the Brenner region, however, the northern Simplon footwall shows strike‐slip dominated postmylonitic deformation related to slip along the Rhône Line. The footwalls of these detachment systems define a unique class of core complex characterized by the development of a midcrustal, synthetic lower hinge that acts to steepen the detachment system and accommodate footwall uplift by a subvertical simple shear mechanism. Both systems have poorly developed upper hinges that flatten the middle to upper crustal ramp by only a few degrees of dip, which is in contrast to detachment fault systems of the western United States, where evidence of a deep hinge is rarely observed, and the upper hinge acts to deactivate the flattened, updip part of the detachment. Our results emphasize the role of tectonic setting buoyancy forces, and synkinematic fluids in controlling the observed style and mechanisms of large‐magnitude extension during collision between continental masses.

Rheological control on the initial geometry of the Raft River detachment fault and shear zone, western United States

The strain, exhumation history, and field orientation of a well‐exposed shear zone and detachment fault in the Raft River Mountains of northwestern Utah, a Cordilleran metamorphic core complex, have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault. Mapping and strain and kinematic analysis indicate that the top‐to‐the‐east Raft River shear zone initially developed parallel to an unconformity separating Archean rocks from overlying Proterozoic quartzite and schist for at least 24 km in the shear direction. Experimental rock deformation data from lithologies similar to the Archean and Proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures; the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below. An extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature‐lateral distance coordinates. The initial average dip of the shear zone and basement unconformity is estimated between 7° and 30°, assuming subhorizontal isotherms and geothermal gradients of 20°–40°C/km. The east dip of the unconformity at the onset of Miocene extension is interpreted to have resulted from late Eocene unroofing and flexure beneath a top‐to‐the‐WNW extensional shear zone in the western Raft River, Grouse Creek, and Albion Mountains. The observations from the Raft River shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but, rather, the orientation of rheological boundaries along which deformation is localized. Furthermore, detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite, and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock. The common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation. Because of this preference, studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal‐sense shear fractures (faults) within the seismogenic crust.

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.

Kinematic linkage between internal zone extension and shortening in more external units in the NW Alps

In the Alps a major extensional shear zone unroofed eclogites in its footwall from 30 kbar to c. 10 kbar, implying of the order of 60 km of relative vertical movement in the period 45–36 Ma. The horizontal component of divergence was of the order of 100 km. We use the foreland basin sedimentary record to prove that the extension was contemporaneous with thrusting in units structurally beneath the eclogites (the Briançonnais and/or External Zones). This thrust-related convergence during that time period was of the order of 44–56 km and therefore of the same order of magnitude as internal zone divergence. Extension may have been driven by internal density contrasts. Buoyant continental crust underneath the mafic eclogites, or dense subducted slab offset from the orogen, can induce extension. Any dynamic model must account for the comparable magnitudes of extension and shortening in adjacent parts of the orogen: relative plate motions in the period 45–36 Ma played a subsidiary role.

Devonian Extension in Northwestern Newfoundland: 40Ar/39Ar and U‐Pb Data from the Ming’s Bight Area, Baie Verte Peninsula

Abstract The Ming’s Bight Group of northwestern Newfoundland, an outlier of Humber Zone continental margin rocks, is entirely surrounded by ophiolitic rocks of the Dunnage Zone. Structures in the Ming’s Bight Group and adjacent units record three main phases of deformation. The earliest structures relate to Silurian sinistral transpression previously documented in the region. Two later phases of extensional deformation produced a series of dextral oblique‐normal shear zones and faults that now separate the Ming’s Bight Group in the footwall from ophiolitic and granitoid rocks in the hangingwall. 40Ar/39Ar and U‐Pb data constrain the times of oblique‐normal shear and cooling. Metagabbro in the Point Rousse Ophiolite Complex, which lies in the hangingwall, preserves disturbed Ordovician hornblende 40Ar/39Ar ages, whereas adjacent shear zones record Devonian ages. Hornblendes in Pacquet Harbour Group amphibolites within extensional shear zones mainly record 40Ar/39Ar ages of 390–380 Ma. Synkinematic titanite...

Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc

Combined structural and petrographical investigations, coupled with 40Ar/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure‐temperature (P‐T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat‐lying ductile to brittle extensional shear zone. The upper complex consists of a nappe‐like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite‐bearing sequence showing typical high‐P/low‐T parageneses (Mg‐carpholite and Na‐amphibole). The 40Ar/39Ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene‐Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks, reworking the previous nappe contacts with shear localization along the upper/lower tectonic complex discontinuity. The 40Ar/39Ar dating indicates that this postnappe stacking tectonic evolution took place from 30 Ma onward. It is proposed that exhumation of the deep‐seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachment is bracketed between 30 Ma and the post‐orogenic Neogene basin sedimentation (middle‐upper Miocene). The revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high‐P/low‐T top to the east crustal thickening episode during the construction of the Apennine orogenic wedge (Eocene‐Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the late Oligocene.

A 2.44 Ga syn-tectonic mafic dyke swarm in the Kolvitsa Belt, Kola Peninsula, Russia: implications for early Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield

The Kolvitsa Belt in the south-western Kola Peninsula formed coeval with the earliest Palaeoproterozoic rift-belts in the Fennoscandian Shield. The Palaeoproterozoic history of this belt comprises the deposition of the 2.47 Ga Kandalaksha amphibolite (metabasalt) sequence onto 2.7 Ga granitoid gneisses, the intrusion of the 2.45–2.46 Ga Kolvitsa Massif of gabbro-anorthosite and the subsequent multiple injection of mafic dykes and magmatic brecciation, followed by the intrusion of 2.44 Ga dioritic dykes, and extensive shearing at 2.43–2.42 Ga. The gabbro-anorthosite and dykes contain high-pressure garnet-bearing assemblages that have previously been considered as evidence for metamorphism in a compressional setting of the Kolvitsa Belt at 2.45–2.42 Ga, i.e. coeval with the formation of the Imandra–Varzuga rift-belt and layered mafic intrusions in an extensional setting. The Kochinny Cape study area on the White Sea coast presents an unique remnant of a 2.44 Ga mafic dyke swarm that endured ca. 1.9 Ga collision but preserved its primary structural pattern well. All these dykes were intruded along numerous NW-trending shear zones within the Kolvitsa Massif and contain angular xenoliths of sheared gabbro-anorthosite. Every new batch of mafic melt underwent shearing during or immediately after solidification, and later dykes intruded into already sheared dykes. Thus, rocks of the Kolvitsa Massif and its dyke complex were successively injected into a large-scale shear zone which was active from ca. 2.46 to 2.42 Ga. Multiple injection of mafic melts, the presence of mutually intruding, composite, sheared mafic dykes, of magmatic breccias with gabbroic groundmass, and of host rocks fragments (showing no evidence of tectonic stacking at the time of brecciation), all indicate an extensional setting. Shearing was also extensional as it occurred simultaneously with the multistage magmatism. The asymmetric morphology of deformed dykes, and asymmetric flexures within weakly deformed lenses show that all these extensional shear zones, apart from a few exceptions, are dextral, were formed in a transtensional setting and are attributed to general W–E to WSW–ENE extension. Structural data available for 2.4–2.5 Ga magmatic rocks elsewhere in the Kola region suggest that the same kinematics operated on a regional scale. The presence of the garnet-bearing assemblages in gabbro-anorthosite and dykes may be explained by crystallisation and shearing of the magmatic rocks at deep crustal levels. Alternatively, corona development might have occurred much later as a result of tectonic loading due to the juxtaposition and overthrusting of the Umba Granulite Terrane onto the Kolvitsa Belt at ca. 1.9 Ga. In view of the field evidence and published ages, an overall extensional setting rather than a combination of compressional and extensional zones is preferable for Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield at 2.5–2.4 Ga.

Age, evolution and regional setting of the Palaeoproterozoic Umba igneous suite in the Kolvitsa–Umba zone, Kola Peninsula: constraints from new geological, geochemical and U–Pb zircon data

The Umba igneous complex consists of an enderbite–charnockite suite, including porphyritic variety of charnockites, and a porphyritic granite. Both are intruded by irregular veins or minor bodies of later reddish granite. The porphyritic charnockites locally contains abundant xenoliths of country rocks and its contamination by sedimentary material is expressed by a minor content of garnet, that increases in amount in areas with sedimentary inclusions. The Umba igneous complex and the Umba block metasediments were deformed together during two episodes of deformation. The first one was a major episode of thrusting with the formation of a penetrative shear foliation ( S 1), which dips gently eastwards, and a gently SE-plunging lineation. Coeval with this thrusting, the boundary between the Umba block and the Poriya Guba series in the southeast developed as a strike-slip shear zone, that juxtaposed the two blocks along a tectonic melange zone. The S 1-shearing deformed the enderbite–charnockite suite, and probably also the porphyritic granite, into plate-like, eastward-dipping bodies. Predating the shearing, the metasediments underwent high-grade metamorphism and anatexis leading to a high degree of partial melting. This anatexis is also found in the enderbite–charnockite suite, but in a much smaller scale and mainly in the marginal parts of the bodies. The second episode of deformation formed narrow localized extensional shear zones ( S 2), which are developed in all rock units. The S 1-shearing in the tectonic melange zone occurred under high-pressure metamorphism during cooling at constant pressure ( T=806–818°C, P=9.3–9.5 kbar) and then at decreasing pressure due to tectonic uplift. Both seem to have gone through the same deformation events as the metasediments. The S 2-extension occurred under decompression ( P=7.5–8.0 kbar, T=860–840°C) caused by uplift or tectonic erosion of the thrust pile. Though indistinguishable in the field the enderbite–charnockite suite form a discontinuous suite with a trondhjemitic trend for the former, and a calc–alkaline trend for the latter. Geochemical study shows that the charnockite group is more strongly differentiated than the enderbite group and that magmatic differentiation in the charnockites were controlled by K-feldspar fractionation. The enderbites, on the other hand, lack differentiation and are considered to have crystallized rapidly from its magma source. The charnockites came from a different source that, judging from the high K/Rb ratio, formed at a deeper crustal level than the enderbites. Both members of the enderbite–charnockite suite formed due to subduction in an island arc setting, and Sm–Nd model ages of 2.1–1.9 Ga indicate that the Palaeoproterozoic suite has a juvenile character. Conventional U-Pb zircon dating of the porphyritic charnockite has given discordant ages of 1912.5±7.7 Ma, 1949±7 Ma and 1966±9. Our preferred interpretation is that the 1912.5±7.7 Ma age represents the age of intrusion, or maximum intrusion age of the charnockites, and that the 1949±7 Ma and 1966±9 Ma ages for the abraded type represent ages or mixed ages of inherited zircons from the contaminating Umba block metasediments. The youngest detrital zircons in these metasediments have similar ages. Their source could have been early magmatic arc intrusives, which were eroded shortly after their formation. If the Umba metasediments were deposited in a magmatic arc setting their initial deformation in an evolving arc may have provided the necessary heat flow for anatexis and high-grade peak metamorphism of the metasediments. Therefore, the intrusions of the enderbite–charnokite suite during the later evolution of the magmatic arc could have post-dated the peak of metamorphism, but still pre-date collision and thrusting leading to tectonic telescoping of the units, and thus explain the lower metamorphic grade in the Umba igneous complex compared to the metasediments. If the 1912.5±7.7 Ma age represents the maximum time of intrusion, the true intrusion age might be slightly younger.