Strona-Ceneri Zone Research Articles

Cadomian to Cenerian accretionary orogenic processes in the Alpine basement: the detrital zircon archive

New whole-rock geochemical and detrital zircon U–Pb and Lu–Hf data of metasedimentary sequences of the Silvretta Nappe, Orobic Alps, Strona-Ceneri Zone, Gotthard Massif and Venediger Nappe are presented. These units seem to share a common early to middle Paleozoic geological record, which has alternatively been interpreted as the result of intraplate or orogenic processes. Detrital zircon data mainly indicate late Ediacaran to early Ordovician maximum sedimentation ages for the studied sequences, suggesting that they were intimately related to Cadomian and Cenerian orogenic processes along the northwestern Gondwana margin. The common presence of late Ediacaran to Cambrian Cadomian ages associated with variable subchondritic to suprachondritic Lu–Hf compositions points to recycling processes of Cadomian sequences, further supported by geochemical data indicating a relatively low to moderate maturity of sedimentary protoliths. The occurrence of Cenerian arc-related intrusions in Austroalpine and South Alpine basement unit points to an arc/back-arc position in the early Paleozoic Cenerian orogen, except for the Strona-Ceneri Zone, which was likely located closer to the forearc region, as indicated by the presence of high-pressure metamorphism. Younger sequences, such as the Landeck Quartz-phyllite, document post-Cenerian sedimentation, whereas those of the Venediger Nappe more likely record the early stages of Variscan subduction, as indicated by Devonian maximum deposition ages.

Tracing the cryptic Sardic (Ordovician) metamorphism across Alpine Europe: the Krndija region in the Slavonian Mountains, Croatia

Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian.

Vestiges of Cambro-Ordovician continental accretion in the Carpathian-Balkan orogen: First evidence of the ‘Cenerian event’ in the central Serbo-Macedonian Unit

In the Balkans, the Serbo-Macedonian Unit (SMU), Serbia, is thrust bounded by the composite Tethyan Vardar Zone and the Carpatho-Balkanides. The SMU actually emerges from beneath the Neoalpine Miocene–Pliocene deposits. Both provenance and geodynamic position of the SMU are poorly known and still debated. This paper reviews the data hitherto published and includes some new field data interpretations. The SMU is composed of a Neoproterozoic–Cambrian high-grade (para- and ortho-) gneiss with peraluminous magmatic arc components (560–470 Ma). The SMU is in the contact with Neoproterozoic upper Ordovician–Carboniferous low-grade metasedimentary succession of an accretionary wedge assembly represented by the Supragetic basement. The SMU basement became folded, sheared and metamorphosed around 490–450 Ma. Paleomagnetic data point to high southern latitudes and a peri-Gondwanan position of the SMU at that time, which concurs with glaciomarine evidence recorded from the upper Ordovician sediments at the base of an accretionary wedge succession. Based on the published data and field survey in the Stalać region, we correlate the SMU with the pre-Mesozoic gneiss terrane exposed in the Strona-Ceneri zone of the Alps. This terrane, identified as the Cenerian orogen of the Alaskan subduction type, developed at an active margin of Gondwana during middle Ordovician times. The SMU basement, with augen and migmatitic gneisses and arc-related peraluminous magmatic bodies, developed at this margin as part of the Cenerian belt or its equivalent. Such an orogenic edifice proved transient and in the earliest Silurian the SMU fragments drifted away being bound for Baltica (amalgamated Moesian microplate and Danubian terrane) to which they became accreted in the Carboniferous and included in the southern European branch of the Variscan orogen (Marginal Dacides/Carpatho-Balkanides). Despite considerable Variscan and Alpine reworking, the pre-Variscan, Cenerian-type crustal assembly along with an inferred boundary between the magmatic arc and the accretionary wedge, accompanied by back-arc/forearc deposits, are still decipherable in the Western Balkan countries.

Banded amphibolites in the Alps: a new interpretation in relation to early Paleozoic peraluminous magmatism

The Strona-Ceneri Zone is located south of the Insubric line, where Alpine overprint is mainly brittle and of low grade or even absent. Apart from the unmetamorphic Permo-Carboniferous sediments and intrusiva the Strona-Ceneri Zone is an Ordovician gneiss complex composed of paragneisses, migmatites, peraluminous orthogneisses and banded amphibolites associated with meta-gabbros and meta-ultramafics. Despite of the individual characters of the other Ordovician gneiss terranes north of the Insubric line, most prominently the Aar Massif, the Gotthard, Silvretta and Ötztal nappes, they show analogous lithological formations and pre-Mesozoic structures indicating similar genetic processes. A revision of geological maps and new field observations in these gneiss terranes indicate characteristic spatial relationships of banded amphibolite formations with migmatites and orthogneisses. The contrasting chemistries of the peraluminous rocks (para- and orthogneisses) and the basaltic amphibolites with a lack of intermediate lithologies in between can be explained by the setting of peraluminous arc magmatism within a subduction–accretion complex. In this model the amphibolites represent primary basalts which ponded at the base of the subduction–accretion complex and delivered the heat for the production of peraluminous melts. Volume estimations indicate a “zone of intermingling” composed of immiscible basalts and peraluminous melts with a total thickness of several kilometers. In this “zone of intermingling” the protoliths of the banded amphibolite formations were generated. Steep strike-slip faults, which are important structures for the syn-magmatic cratonization of subduction–accretion complexes, provide pathways for the emplacement of magmas, migmatites and intermingled materials. This results in the formation of steeply oriented sheets of orthogneisses, deformed migmatites and banded amphibolite formations, respectively. Finally, the paper lists many other peri-Gondwanan regions with an early Paleozoic peraluminous arc magmatism, indicating a similar setting on a global scale.

Ordovician orogeny in the Alps: a reappraisal

An important part of the pre-Mesozoic basement of the Alps is the product of orogenic processes occurred in Ordovician times. Most of the geological constrains for this reconstruction have been obtained by the study of the Strona-Ceneri zone in the Southern Alps, where Alpine overprint was weak. The erosion of late Pan-African belts delivered large amounts of greywackes and pelites into subduction zones along Gondwana. The sediments were subducted and accreted to form wide complexes. As a result of the large sediment input, subduction retreated and the mantle-derived magmas intruded the base of the fertile “mud pile.” This initiated substantial anatexis to produce peraluminous magmas, which intruded and extruded syntectonically. Thereby, the predominantly steeply structured subduction–accretion complex provided ideal pathways for the uprising magmas, and down-thrusting host rocks to result in an isostatically stable crust. The SE Australian Lachlan fold belt is interpreted as an upper crustal analog of the Strona-Ceneri zone. Based on the combination of their geology, a crustal profile through an Alaskan type of orogen is drawn.

Thermochronological constraints on the multiphase exhumation history of the Ivrea-Verbano Zone of the Southern Alps

The Ivrea-Verbano Zone of the western Southern Alps (NW Italy) exposes a well-preserved tilted section across the lower continental crust, making it a key region for studying deep crustal and exhumation processes. This paper refines the cooling and exhumation history of the Ivrea-Verbano Zone using K/Ar dating of mica and illite-rich fault gouges as well as zircon fission track and (U-Th)/He thermochronology. The adjacent Strona-Ceneri Zone, Sesia-Lanzo Zone and Lower Penninic nappes are included in the study to derive a broader picture of the low-temperature history of the area.In the Strona profile of the Ivrea-Verbano unit the biotite K/Ar, zircon fission track and (U-Th)/He geochronometers show well preserved, but unusually wide partial retention zones. The youngest ages, representing the formerly deepest position, are situated along the Insubric Line.The main foliation of the Ivrea-Verbano Zone is vertical. The exhumation of the Ivrea-Verbano Zone, which section has a horizontal position on the surface now – took place in three steps. During Jurassic time the Ivrea-Verbano Zone was exhumed to a shallow to mid-crustal position by continental-scale extension. In this displacement the Pogallo Line probably played a dominant role. The studied section occupied an oblique position with a calculated angle of ca. 15 to 23° in the Jurassic. Later the Ivrea-Verbano Zone experienced a minor cooling event in the Late Eocene (~38Ma zircon fission track ages) that was probably related to thrusting and erosion. The final exhumation towards the surface took place in the mid-Miocene as documented by the ca. 14Ma zircon (U-Th)/He ages and a 12.8MaK/Ar fault gouge age. The magnitude and the high rate of final exhumation suggest orogen-parallel extension as a driving force, which is widespread in the Alps in the Lower to Middle Miocene and is most probably connected to orogenic collapse.

Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps

Abstract The Ivrea Zone (southern Alps) is one of the key regions interpreted as exposing a section of the lower continental crust and was the subject of several review-type articles. The Ivrea–Verbano Zone was rotated into an upright position along the Insubric mylonite belt. In the southeast, this unit is in contact with the Strona Ceneri Zone, which is interpreted as upper continental crust crossing the Permian Cossato–Mergozzo–Brissagio Line (CMB Line). The CMB mylonites are locally overprinted by the mylonites and cataclasites of the Pogallo Line, which was active during the Jurassic. In addition, the sinistral, steeply inclined Rosarolo shear zone was active over a long time span from the ductile into the brittle field, i.e. from the Early Permian (high-temperature ultra-mylonites) to the Neo-Alpine basic dykes and pseudotachylites. The high-temperature mylonites accommodated crustal extension and may be related to normal faults generated by magmatic underplating. The reactivation at different crustal levels during exhumation and tilting is documented by strain increments at decreasing P/T conditions. Its present subvertical orientation was attained during the Neo-Alpine deformation. Constraints on its exhumation history are based on new 40 Ar/ 39 Ar hornblende ages, K–Ar biotite ages and zircon fission-track data along the NE–SW trending Valstrona section. A re-interpretation of existing U–Pb monazite ages is included, based on a higher closure temperature for monazite. The oldest monazite ages are observed in proximity to the Pogallo Line ( c . 292 Ma). Heat input by mafic intrusions was sufficient to reset the U–Pb monazite system, as is evidenced by the youngest ages in the vicinity of the Insubric Line. The re-interpretation favours the hypothesis that the oldest monazite ages are the result of complete resetting by a Permian thermal event. The 40 Ar/ 39 Ar hornblende ages and K–Ar biotite ages document the cooling after Permian heating. Roughly parallel age progressions decrease from the Pogallo Line (hornblende: 271 Ma vs. biotite: 227 Ma) towards the Insubric Line (hornblende: 201 Ma vs. biotite: 156 Ma). Zircon fission-track ages run parallel to the biotite ages in the upper part of the profile, whereas towards the Insubric Line a significant deviation from the biotite age progression is attributed to tilting of the basement during the Oligocene. Zircon fission-track ages around 38 Ma are found close to the Insubric Line. No age offset, neither at the CMB nor at the Pogallo Line, is observed. This confirms the hypothesis that the Pogallo Line is an oblique normal fault, and that the CMB Line has accommodated only minor vertical displacement. The capture of the different cooling ages confirms the tilting of the Ivrea–Verbano Zone during the Neo-Alpine deformation and contradicts the tilting of the Ivrea–Verbano Zone during the Permian.

Caledonian high-pressure metamorphism in the Strona-Ceneri Zone (Southern Alps of southern Switzerland and northern Italy)

The Strona-Ceneri Zone comprises a succession of polymetamorphic, pre-Alpidic basement rocks including ortho- and paragneisses, metasedimentary schists, amphibolites, and eclogites. The rock pile represents a Late Proterozoic or Palaeozoic subduction accretion complex that was intruded by Ordovician granitoids. Eclogites, which occur as lenses within the ortho-paragneiss succession and as xenoliths within the granitoids record a subduction related high-pressure event (D1) with peak metamorphic conditions of 710 ± 30 °C at 21.0 ± 2.5 kbar. After isothermal uplift, the eclogites experienced a Barrowtype (D2) tectonometamorphic overprint under amphibolite facies conditions (570-630 °C, 7-9 kbar). U-Pb dating on zircon of the eclogites gives a metamorphic age of 457 ± 5 Ma, and syn-eclogite facies rutile gives a 206Pb/238U age of 443 ± 19 Ma classifying the subduction as a Caledonian event. These data show that the main tectonometamorphic evolution of the Strona-Ceneri Zone most probably took place in a convergent margin scenario, in which accretion, eclogitization of MOR-basalt, polyphase (D1 and D2) deformation, anatexis and magmatism all occurred during the Ordovician. Caledonian high-pressure metamorphism, subsequent magmatism and Barrow-type metamorphism are believed to be related to subduction and collision within the northern margin of Gondwana.

Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic.

Seismic structure of gneissic/granitic upper crust: geological and petrophysical evidence from the Strona-Ceneri Zone (northern Italy) and implications for crustal seismic exploration

We present a stochastic seismic model of gneissic/granitic upper crust by estimating the amplitude probability distribution and the autocorrelation function of the inferred seismic velocity field of the Strona-Ceneri Zone (northern Italy). Unlike previous investigations of the adjacent Ivrea Zone and the Lewisian gneiss complex (Scotland), the lithologies mapped in the Strona-Ceneri Zone cannot be directly correlated with laboratory-measured seismic velocities. The amplitude probability distribution of the extensive petrophysical database and the relatively uniform mineralogical composition of the lithologies indicate that the velocity distribution in the Strona-Ceneri Zone can be approximated as being continuous and Gaussian. Based on this assumption and using the spatial autocorrelation function measured from a digitized geological cross-section, we have modelled the fine-scale structure of the Strona-Ceneri Zone as a self-affine medium with a fractal dimension of 2.50 ± 0.15 and horizontal and vertical characteristic scales of 2.6 ± 0.4 km and 1.4 × 0.3 km, respectively. On near-offset shot-gather seismic data, Strona-Ceneri-type upper crustal heterogeneity produces a reflected field with the appearance of incoherent scattered noise characteristic of non-reflective, ‘transparent’ upper crust found in extended Phanerozoic provinces. This type of heterogeneity has a considerable effect on seismic imaging of underlying reflective lower crust: individual reflection segments are disrupted and shifted in time by up to several tenths of a second, and the boundaries of the reflective zone become distorted and blurred. At far offsets seismic scattering from heterogeneity of this type reproduces the commonly observed rapid decay and scattered distribution of Pg amplitudes. Comparable amplitude decay patterns result from layered deterministic upper crustal velocity structures with velocity inversions.

Seismic properties of rocks from an exposure of extended continental crust—new laboratory measurements from the Ivrea Zone

Compressional wave velocities for new samples from the Ivrea-Verbano Zone (IVZ) and Strona-Ceneri Zone (SCZ) have been measured to 600 MPa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains. These data, coupled with previous measurements, show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of SCZ and IVZ schists and gneisses. IVZ paragneisses form the basis of a linear velocity-density relationship for metagraywackes of various metamorphic grade. This relationship falls close to that expected for mafic rocks and overlaps velocity-density data for IVZ mafic gneisses. One-dimensional near-vertical reflection models, based on these data and recent structural reconstructions of the terrain, show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite-granulite facies paragneiss, mafic gneiss and ultramafic rocks. Transparent upper crustal levels in the model result from the overall similarity of velocities for schists, gneisses and granites from the structurally and lithologically complex SCZ. A mean lower crustal velocity of 7.1 km/s is calculated for the part of the IVZ composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex. The velocity-density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from P-wave velocity data in the absence of other constraints.

Tectonometamorphic history of the Ivrea Zone and its relationship to the crustal evolution of the Southern Alps

The Ivrea Zone represents a cross-section through the lower continental crust of the Southern Alpine basement. A first long episode with regional metamorphism and associated polyphase deformation ended in the Variscan. Subsequent Late Paleozoic magmatic activity may have occurred during an early stage of crustal attenuation. Late Paleozoic and Early Mesozoic crustal thinning is accommodated by conjugate high-temperature shear zones within the granulite facies Ivrea Zone and by low-angle normal faulting within the Pogallo Ductile Fault Zone, at the base of the intermediate crust. The age and kinematics of the Pogallo Ductile Fault Zone are consistent with the occurrence of Early Mesozoic extensional basins in the Southern Alpine sediments. Exhumation and final steepening of the Ivrea Zone during the Alpine Orogeny did not substantially alter its internal structure exept in the vicinity of the Insubric Line. Thus, the Ivrea Zone and the adjacent Strona-Ceneri Zone represent a good example of highly attenuated lower and intermediate continental crust.

The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone

Schmid, SM., Zingg, A. and Handy, M., 1987. The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone. In: H.J. Zwart, M. Martens, I. van der Molen, C.W. Passchier, C. Spiers and R.L.M. Vissers (Editors), Tectonic and Structural Processes on a Macro-, Meso- and Micro-Scale. Tecta~a~hys~~, 135: 47-66. The Insubric Line west of Locamo is characterized by a 1 km thick greenschist facies mylonite belt. East of Locamo, these mylonites are overprinted by a discrete brittle fault. The mylonites are derived from basement units of the Central Alps (Sesia Zone) and the Southern Alps (Ivrea Zone) as well as from the Permo-Mesozoic cover of the Southern Alps (Canavese). Sense of shear criteria indicate that the mylonites accomm~ated backt~s~g followed by dextral strike-slip motion. Mylonitization during backtbrusting was synchronous with backfolding of the Central Alpine nappes under higher metamorphic conditions. A horizontal temperature gradient resulted from the rapid juxtaposition of the warm Central Alpine block against the cold Southern Alpine block. Mylonites formed during the later dextral strike-slip event are related to large transcurrent displacements in the Central Alps deduced from regional kinematic considerations. Thus, both mylo~t~ation events are contempor~eous with deformation to the north and south of the Insubric Line (Insubric phase) extensively modifying the pre-Insubric crustal configuration of the Alps. The Insubric phase post-dates the Bergell intrusion (30 m.y.). The emplacement of the geophysical Ivrea body is a combined effect of vertical uplift due to E-W directed crustal thinning during the Early Jurassic and underplating by continental crust associated with Late Cretaceous compression. A deep crustal normal fault (Pogallo Line), subsequently rotated during Tertiary Alpine orogenesis, separates deeper parts of the Southern Alpine crust (Ivrea Zone) from intermediate crustal levels (Strona-Ceneri Zone). The rigid Ivrea body localir.ed large strains within the Insubric mylonite belt and is responsible for the present curvature of the Insubric Line. In~~uetion

Pogallo Line, South Alps, northern Italy: An intermediate crystal level, low-angle normal fault?

The Pogallo Line is a major crustal shear zone in the crystalline South Alps of northern Italy which separates middle amphibolite–granulite facies rocks of the Ivrea zone from lower–middle amphibolite facies rocks of the Strona-Ceneri zone. U-Pb, Rb-Sr, and K-Ar whole-rock and mineral ages for samples from this region indicate that the two zones pursued different time-temperature paths after Caledonian peak metamorphism. Cooling curves for the two terrains finally converge in the Late Triassic(?)–Early Jurassic, a time of large-scale crustal extension in the South Alps that anticipated the opening of the western Tethys. We suggest that the Pogallo Line may represent a low-angle normal fault that developed at intermediate crustal levels during this period of extension and attained its present subvertical orientation during Alpine deformation.