Thrust Units Research Articles

Strain partitioning in the footwall of the Somiedo Nappe: structural evolution of the Narcea Tectonic Window, NW Spain

The Somiedo Nappe is a major thrust unit in the Cantabrian Zone, the external foreland fold and thrust belt of the North Iberian Variscan orogen. Exposed at the Narcea Tectonic Window are Precambrian rocks below the basal decollement of the Somiedo Nappe, which exhibit a different deformation style than the overlying Paleozoic rocks above the basal decollement. During Variscan deformation, folding and widespread subhorizontal, bedding-parallel decollements were produced in the hanging wall within the Paleozoic rocks. Vertical folding, with related axial-planar cleavage at a high angle to the decollement planes, developed simultaneously in the upper Proterozoic Narcea Slates of the footwall, below the detachment. The relative magnitude of finite strain, measured in the footwall rocks, diminishes towards the foreland. These observations indicate that (1) significant deformation may occur in the footwall of foreland fold and thrust belts, (2) the shortening mechanism in the footwall may be different from that of the hanging wall, and (3) in this particular case, the partitioning of the deformation implies the existence of a deeper, blind decollement surface contemporaneous with the first stages of the foreland development, that does not crop out in the region. This implies a significant shortening in the footwall, which must be taken into account when restoration and balancing of cross-sections is attempted. A sequential diagram of the evolution of the Narcea Tectonic Window with a minimum shortening of 85 km is proposed, explaining the complete Variscan evolution of the foreland to hinterland transition in the North Iberian Variscan orogen.

Kinematics of primary contacts between low- and relatively high-pressure rocks in orogens

Primary contacts among low- and relatively high-pressure rocks in convergent orogens have been mainly interpreted as a result of extensional tectonics, or reimbrication of high-pressure thrust units already resting on low-pressure ones and by consequent inversion of their stacking order. Our work in northern Calabria allows us to ascertain the nature and kinematics of several of these contacts, and to work out a general model that considers primary pressure break contacts as resulting from the growth of a deep passive roof duplex within a subduction-accretion system. Estimates of pressure differences across these contacts may be obtained for a simple passive roof duplex geometry as a function of the aspect ratio of the structure and of the angle of subduction. Our results suggest therefore that there may be a further possibility, in addition to extension or reimbrication, which explains pressure break contacts in orogens.

Compressional and extensional tectonics in an arc system: example of the Southern Apennines

Adopting the Southern Apennines as a case example, we determine the relationships between extensional and compressional structures within an arc system, based on tectonic analysis in the field. We show that, in addition to the well-known occurrence of inner extensional structures during frontal accretion, two other types of extensional structures are present in the Southern Apennines. We classify the three types of extensional structures depending on their chronologies with the accretion of the thrust unit, and we characterize them in terms of palaeostresses. Some normal faults of the mountain chain are old extensional structures formed in a foreland basin. In the Matese Mountains, such faults with offsets ranging several hundred metres are sealed by a late Neogene thrust sheet. These foredeep structures were created by extension trending almost perpendicular to the axis of the present-day foreland basin. Other normal faults, located closer to the Tyrrhenian sea, developed simultaneously with the Pliocene-Quaternary accretion of thrust sheets at the front of the belt. In the Salerne graben (a Tyrrhenian Basin structure), extension occurred perpendicular to the compression that prevailed at the front of the mountain belt. A stress gradient existed between the outer part of the arc (with σ 1 trending ENE-WSW) and the inner part (with σ 2 trending ENE-WSW). In the Southern Apennines, middle Pleistocene-Holocene normal faults post-date all compression and thrusting. The corresponding recent stress field (NE-SW extension) results from uplift of the previously subsiding Adriatic lithosphere. We conclude that a given thrust unit of this NW-SE-trending mountain chain may have successively undergone NE-SW extension in the foreland basin, NE-SW compression during the accretion, NW-SE extension in the back-arc region and NE-SW extension during the recent evolution of the prism. The succession of these tectonic regimes has induced complex structures, as commonly observed in an arc system.

Thrust tectonics in the Picentini Mountains, Southern Apennines, Italy

Detailed geological mapping carried out in the Picentini Mountains, Southern Apennines, Italy, allowed to reconstruct the geometry of the fold and thrust belt in this region. Contractional structures were formed during multiple episodes of ENE to NNE shortening and were cut during later extension by low- and high-angle normal faults. Based on timing of emplacement and geometrical relationships between thrust units, we worked out a kinematic model of thrust tectonics. Basinal (Sicilid and Lagonegro) units were thrust eastward onto a carbonate-platform-basin system (CPBS) starting in the Serravallian-Tortonian, and were in turn overridden by the CPBS units by means of deeper decollement thrusts. Later contraction, starting from late Tortonian-Messinian times, built up a 15-km-thick antiformal stack during SSW-NNE shortening. We applied the forward kinematic model of thrust imbrication to perform a qualitative palinspastic restoration of a regional cross-section through this area, based on published interpreted seismics and other subsurface data. The thrust tectonics of the Picentini Mountains and more northward regions was controlled in the internal sectors mostly by envelopment thrusting of carbonate platform thrust sheets, which formerly were the floor complex of the Tortonian thrust belt, while multiple progressive decollement of the basinal roof complex occurred in the external part of the belt. Shallow crustal extension on low-angle faults with transport direction oblique to orthogonal to contractional transport was responsible for contemporaneous thinning during the accretion of the antiformal stack at deeper structural levels.

Pre‐Neogene evolution of the Western Carpathians: Constraints from the Bochnia‐Tatra Mountains section (Polish Western Carpathians)

On the basis of field studies and subsurface data, this first balanced transect crosses most of the Carpathian foreland basin and the entire Polish Western Carpathian thrust units (Outer flysch nappes, Pieniny Klippen Belt, and Inner Carpathian Tatric thrust sheets). It shows that (1) me Outer Carpathians are made up of several flysch units whose internal structure is strongly controlled by initial thicknesses, whereas the thin flysch successions are strongly deformed by tight folds and include several imbricated thrust sheets; by contrast, the thick flysch successions are less deformed and accreted in slightly imbricated thrust sheets; and (2) the Inner Carpathians are structured in an antiformal stack of basement thrust sheets. The unfolding of this cross section shows that the post‐middle Oligocene shortening in the European foreland platform and Outer Carpathians exceeds 180 km. In the substratum, most of this shortening seems to be solved by basement consumption vertically beneath the suture zone of the Pieniny Klippen Belt. Additional basement shortening took place also (1) in the Inner Carpathians, where, during the Neogene, buried basement thrust sheets were piled up to form an antiformal stack, and (2) beneath the inner zones of the Outer Carpathians. Palinspastic restoration of the cross section reflects two well‐differentiated zones prior to the late Oligocene‐late Miocene compression: (1) the present Inner Carpathians and the Pieniny Klippen Belt, characterized by a Late Cretaceous thrust belt unconformably overlain by an undeformed Palaeogene cover, and (2) the Outer flysch Carpathians, in which pre‐upper Oligocene configuration comprised a typical foreland basin flexed toward the SSW, with a system of NW trending highs and troughs.

Rotational deformation during Palaeogene thrusting and basin closure in eastern central Greece: palaeomagnetic evidence from Mesozoic carbonates

SUMMARY Palaeomagnetic studies in Greece have concentrated upon Neogene sequences, with Mesozoic rocks of mainland Greece receiving little attention. The Neogene data base is now sufficiently extensive, however, to allow meaningful interpretation of older units. This paper presents new palaeomagnetic data from Mesozoic and Tertiary sequences of eastern central Greece, which document a pre-Neogene phase cf rotational deformation. Mesozoic carbonates have been sampled within the classic Pelagonian Zone in the Argolis Peninsula, Beotia and Evvia. Pelagonian sequences record the development, drowning and subsequent re-establishment of a carbonate platform separating two strands of the Neotethys Ocean (the Pindos and Vardar basins). Most platformal facies are tdo weakly magnetized for palaeomagnetic analysis. However, pelagic horizons consisting of pink condensed limestones have been deposited periodically in response to platform subsidence. These Middle-Late Triassic, Jurassic and Upper Cretaceous sequences yield high-quality palaeomagnetic data, which reveal a consistent clockwise rotation of about 94. The constancy of rotation angle deduced from the Mesozoic rocks suggests that the total Tertiary rotation is uniform throughout the area studied. This implies that observed variations in declination within the Neogene sequences result from sampling of rocks of different ages deposited during a period of rapid overall rotation. Removal of the largest Neogene rotation (45) from the Mesozoic data set reveals a pre-Middle Miocene, post-Late Cretaceous clockwise rotation of the southern Pelagonian Zone of 50. The major tectonic event during this time period was the closure of the Pindos Ocean during the Eocene. A large carbonate platform within the Pindos basin, now preserved in the Parnassos Zone, presented a major obstacle to the advancing Pelagonian nappes during SW-directed thrusting (present coordinates). Impingement of the nappes on to the Parnassos platform resulted in pinning and subsequent clockwise rotation of thrust units around the south-eastern margin of the platform. The southernmost tip of the Pelagonian Zone (southern Argolis) was detached along a pre-existing line of weakness along the present Migdhalitsa Graben, and experienced a different rotation history. Additional palaeomagnetic data from the Pindos nappes in the eastern Peloponnesos also provide evidence for significant rotation of thrust sheets during Eocene basin closure. The Pindos sequences formed by pelagic sedimentation within the Pindos Ocean. An overall anti-clockwise rotation with respect to the underlying Gavrovo-Tripolitza autochthon is implied by the data. This probably reflects oblique emplacement of the Pindos thrust sheets on to the adjacent platform. Variations in rotation angle probably result from variations in footwall topography.

Late Cretaceous to Palaeogene thin-skinned tectonics of the Palmyrides belt (Syria)

Detailed mapping in the Palmyrides belt (Syria), shows that its structure resulted from thin-skinned tectonics and includes duplexes, anticlinal stacks and imbricate thrusts. In the Frontal Palmyrides the main décollement level is formed by Triassic evaporites but a Cenomanian marl interval locally forms a subsidiary décollement. Ramps linking the two décollement levels generate duplexes formed by thin horses of Jurassic and Lower Cretaceous material. To the north, in the Central Palmyrides, the décollement ramps down from a Triassic to an Eocambrian level, in which it continues as a main basal décollement. The ramp connection between the Cambrian and Triassic décollement levels is responsible for the anticlinal structure of the Central Palmyrides (Bilas block). Accurate mapping, balanced cross-sections and forward-deformation modelling allowed calculation of the amount of shortening of the sedimentary cover: the minimum estimate is 20 km. This independent, field-based result is compatible with previous estimates from a general seismic study of the Cenozoic-Mesozoic series. Due to the presence of folded Neogene deposits, the deformation of the Palmyrides belt was thought to be Miocene-Pliocene in age. However, Eocene carbonates unconformably overlie various thrust units in the Frontal Palmyrides. Reconstruction indicates 15 km of shortening during the late Maastrichtian to upper Eocene and 5 km of shortening since the Late Miocene. Thus, the main shortening event of the Palmyrides occurred during the Palaeocene. Seismic, well and gravity data are used to integrate the shallow structure in a crustal-scale deformation model. Gravity modelling confirms previous interpretations of a relatively flat Moho beneath the Palmyrides belt and suggests that deformation of the belt resulted from a movement of the sedimentary cover of northern Syria 20 km to the south, along a 200 km long, flat décollement within the Eocambrian. This Syrian décollement between the sedimentary cover and the basement is supposed to root down north of Syria, within the North Arabian plate boundary. Such a décollement implies four times more shortening in the sedimentary cover than in the Palmyrides basement.

Upper Jurassic Carbonate Sediments and Reefs in the Carpathian Foreland of Poland

Paleogeographic analyses and plate reconstructions indicate that the Upper Jurassic series of the Carpathian foreland accumulated on the Eurasian shelf of the Tethys Ocean. In the Carpathian foreland of southern Poland, marine Upper Jurassic carbonate series, exceeding 1000 m in thickness, are covered by Tertiary Molasse series or by thrust units of the Outer Carpathians. From a lithostratigraphic point of view, six facies units can be recognized in these Oxfordian to Tithonian carbonates, namely calcareous plankton, calcareous sponge, algal-oolitic, calcareous-marly coquina, dolomitic-calcareous, and upper algal series. During the lower part of the Upper Jurassic, deeper neritic conditions prevailed (calcareous plankton series). These gave way to upward shoaling conditions (calcareous sponge series, deeper basin calcareous-marly series) and a final shallow water environment (algal-oolitic and younger series). Thickness changes of the calcareous-marly series and the diachronous character of the facies boundary between the Oxfordian and Kimmeridgian show that the basin deepened to the south. Barrier reefs and associated fore-reef and back-reef facies occur within the algal-oolitic series. These trend northwest-southeast and reach 30-50 m in thickness. The reefs were mainly built by algae. Algal reefs under the Molasse and Carpathian thrust sheets are potential hydrocarbon exploration targets. The paleogeographic position of themore » Jurassic of the Carpathian foreland lies within the Tethys realm, which contains a large part of the world's producible oil reserves; this lends encouragement to this play.« less

Sandstone-hosted lead-zinc mineral deposits and their relation to the tectonic mobilization of the baltic shield during the Caledonian orogeny?a reinterpretation

Sandstone-hosted lead-zinc impregnation deposits in Scandinavia occur in Vendian to Cambrian and, locally, Ordovician sandstones that rest unconformably on the deeply eroded Baltic Shield. The sandstones are overthrust by the Caledonian nappes or form part of the lowermost Caledonian thrust units. Sandstone-hosted lead-zinc deposits, that occur along the present erosional front of the Caledonides, contain galena and sphalerite cementing fractures and pore space. The deposits formed by fluid-mixing processes in the sandstones. Early ore genetic models considered the ore-hosting sandstones, because of the positive correlation between ore grade and palaeo-permeability, as main aquifer for the metalliferous fluids that were interpreted to be either ground-waters or hot basinal brines driven out from geosynclinal sediments during the Caledonian orogeny.

Crustal-scale balanced cross sections through the Variscan fold belt, Germany: the central EGT-segment

Abstract This paper presents crustal-scale, geometrically and isostatically balanced cross sections through the Rhenohercynian and Saxothuringian zones of the Variscan fold belt of Germany. The cross sections are based on surface geological data and on results of the DEKORP deep reflection profiles. They were constructed by means of computer-assisted, iterative forward modelling. The Rhenohercynian zone, corresponding to the Externides of the Variscan belt, developed out of a some 300 km wide rifted continental margin sedimentary prism, limited to the south by the narrow Giessen oceanic basin. During Carboniferous imbrication this shelf sequence is stripped of its basement above one or more decollements and shortened by some 150 km. Closure of the Giessen ocean and emplacement of the ophiolite bearing Giessen nappe entailed a minimum additional 200 km of crustal shortening. The structure of the model correlates well with the present outcrop pattern of rock units, their metamorphic history, the seismic and magnetotelluric architecture of the crust, and its present regional distribution of thickness. The deeper crust beneath the Rhenish Massif is probably devoid of extremely weak decollement horizons and pronounced rheological stratification. In the southward adjacent Saxothuringian zone, nappe development involving the stacking of crustal slices during Middle Devonian to Visean times accounts for a further 170 km of crustal shortening above an infracrustal, partly blind thrust system with flat-ramp-geometry. Total crustal shortening in the analyzed profiles is estimated to be of the order of 520 km; this figure is a conservative value. Prominent uncertainties include the unknown importance of crustal wrenching, the state of strain within thrust units, the distribution of strain within the lower crust, and major changes in thrusting directions during the course of orogenic history.

Petrology of eclogites and clinopyroxene-garnet metabasites from the Cabo Ortegal Complex (northwestern Spain)

High-pressure, subduction-related metamorphism is recorded in two structural units of the Cabo Ortegal complex of Spain: (i) an upper thrust unit (Concepenido-La Capelada) composed of granulites, eclogites and metaperidotites and (ii) a structurally underlying unit (Chimparra-Banded) formed of gneisses with minor metabasic intercalations. In the upper unit, a metamorphic episode of Lower Ordovician age caused the formation of eclogites from basic rocks with N-MORB compositions, while high-pressure, garnet-clinopyroxene granulites were formed from basic to intermediate protoliths with calc-alkaline affinities. Garnet replaces spinel in ultramafics of the same unit and may have originated simultaneously. The PT-conditions were c. 800°C at 13.5 kbar in the granulites, whereas similar temperatures and higher pressures ( > 17 kbar) prevailed in the eclogites. The migmatitic Chimparra and Banded gneisses contain boudins of garnet and omphacite-bearing rocks, with or without plagioclase. The basic protoliths of these rocks are similar to LREE-enriched basalts with alkaline affinities. The Silurian metamorphism in this gneissic unit took place at c. 700°C or lower and at pressures around 15 kbar. This metamorphism could have occurred when a volcano-sedimentary complex, formed in a thinned-crust environment, was subducted beneath an overlying active margin/volcanic arc complex. An ophiolitic unit, made up of the N- and T-type MORB metabasites of the Candelaria and Purrido-Peña Escrita Formations, underlies the gneisses. Metamorphism in this unit took place during the Early-Middle Devonian, mainly giving rise to amphibolite-facies associations. Sporadic coronitic metagabbros/dolerite and garnet-clinopyroxene metabasites indicate that amphibolite-granulite transitional conditions ( > 700°C, 8 kbar) were attained locally. Retrograde amphibolitic assemblages in the upper units are more or less coeval with this episode, which suggests that it may have been related to the cessation of obduction and the stacking of the overlying high-grade units.

Evidence of an imbricate crustal thrust belt in the southern British Variscides: Contributions of southwestern approaches traverse (SWAT) deep seismic reflection profiling recorded through the English Channel and the Celtic Sea

The Southwestern Approaches Traverse (SWAT) seismic reflection profiles recorded through the Celtic Sea and the English Channel provide unexpected data concerning, notably, the deep structure of the Irish and south British Variscan crust. One of the most significant results is the recognition of prominent deep southerly dipping reflectors, regarded as large‐scale foreland directed Variscan thrusts which constitute, in the southern innermost zones, a crustal stacking wedge. The resulting overthickened crust may have induced the petrogenesis of the Cornubian granitic batholith by an anatectic melting process. The roots of this crustal duplex are not imaged on the SWAT lines because of their subsequent assimilation by the more recent layering of the present‐day lower crust. Upward, the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone, not revealed by the SWAT profiles, and which underlies the thick Devonian allochthonous high‐strained units of Cornwall. Northward, this shallow northerly verging shear zone is overstepped by the Tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as Caledonian features. Southward, the imbricate crustal thrust unit is overridden by the Lizard ophiolitic suture, which appears as a 8‐km‐thick gently southerly dipping sheared zone, crosscuttting the entire crust and rooting deep beneath the present‐day Moho. Its hanging wall is constituted by the Channel Cadomian block, characterized by an almost seismically featureless upper crust which is not involved in the main Variscan thrust stacking. Northward, the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat‐lying decollement level. This frontal ramp emerges along a broadly 110°N trending discontinuous thrust front, located above the northern downfaulted margins of the Munster (Ireland) and Pembroke (Wales) Devonian basins. The SWAT deep seismic data relate the overall Variscan crustal shortening undergone by the south British domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous Channel block and the northern autochthonous foreland. This crustal shortening occurred during a Carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust‐floored basin (Lizard).

Thrust sheet rotation ? — The South Pyrenean Tertiary basin configuration reconsidered

AbstractConcepts of the Alpine structural evolution of the South Pyrenean thrust zone largely rely upon balancing methods applied to cross-sections at right angles to the strike of the orogen.One of the oldest thrust units involved, the Central South Pyrenean (abbreviated : CSP) thrust sheet complex, has a plan geometry of an inverted trapezium with an almost N-S axis of symmetry. Investigations along the western ramp of the sheet and in its nappe-top basin (Tremp-Ager Basin), combined with published data on the frontal ranges and the eastern flank, reveal however a marked asymmetry. Anticlockwise rotational lateral constriction of the Ainsa Basin and a transpressional, left-stepping tear fault system define the western ramp structure until the Middle Eocene. Dextral wrench faults obliquely dissect the frontal thrust folds after the Priabonian (Mediano High, and the Milla Fault). The eastern sector of the sheet is subdivided by a system of thrust faults (Boixols, Montsec thrusts) fanning from the eastern ...

A thin‐skinned tectonics model for an arcuate fold and thrust belt: The Cantabrian Zone (Variscan Ibero‐Armorican Arc)

The Cantabrian Zone constitutes the external zone of the Variscan orogenic belt in the northwest of the Iberian Peninsula. Within it a large number of thrusts and folds can be observed which define the form of the Asturian Arc. Tectonostratigraphically, two units can be distinguished: a pretectonic one and a syntectonic one. The time of the transition between the two units lies close to the Devonian‐Carboniferous boundary. The major allochthonous units of the Cantabrian Zone display varied geometries (duplexes, imbricated thrusts, out‐of‐sequence thrusts, etc.). The allochthonous units were emplaced in a foreland propagating sequence with movement age ranging from Westphalian B to Stephanian. Movement directions changed from one major unit. As a whole, the movement directions converge toward the core of the Asturian Arc, and are best interpreted in terms of a progressive series of rotational displacements, leading to a final disposition of major units similar to that of the leaves of a photographic iris. The folds were initiated at the same time as the motion of each nappe unit, and each type of structure represents the hanging wall adaptation to the development of a lateral and longitudinal staircase thrust surface topography. These folds suffered tightening after the emplacement of the thrust unit in which they occur, as a result of the different direction of movement of the next thrust unit propagating from beneath.

Mud Volcanoes, Shale Diapirs, Wrench Faults, and Melanges in Accretionary Complexes, Eastern Indonesia

In Timor, eastern Indonesia, where the northern margin of the Australian continent is colliding with the Banda Arc, Australian continental margin sediments are being incorporated into an imbricate wedge, which passes northward into a foreland fold and thrust belt. Field mapping in Timor has shown that scaly clays, containing irregularly shaped or phacoidal blocks (up to several meters long) and composed of a wide range of lithologies derived from local stratigraphic units, occur in three environments: along wrench faults, as crosscutting shale diapirs, and associated with mud volcanoes. A model is proposed linking these phenomena. Shales become overpressured as a result of overthrusting; this overpressure is released along vertical wrench faults, which cut through the ove thrust units; overpressured shales containing blocks of consolidated units rise along the fault zones as shale diapirs; and escaping water, oil, and gas construct mud volcanoes at the surface. An extensive melange deposit in Timor, the Bobonaro Scaly Clay, has been interpreted by Audley-Charles and subsequent workers as an olistostrome. Our study interprets the Bobonaro Scaly Clay as the product of shale diapirism. Shale diapirs are likely to be generated wherever water-saturated sediments are incorporated into accretionary complexes. Diapirs have not, however, been recorded as occurring in ancient accretionary complexes, although melanges, usually interpreted as olistostromes, are commonly reported. Criteria are proposed by which the products of shale diapirism might be recognized, including: (1) shape of the diapiric body; (2) its relationship to the surrounding rocks; (3) nature of the matrix and of the enclosed blocks; (4) shapes of the blocks and their relationship to he matrix; and (5) effects of deformation. The importance of shale diapirism in the formation of melanges has not been fully appreciated, and the products of shale diapirism constitute a major component of ancient and present-day accretionary complexes.

Stretching normal to the regional thrust displacement in a thrust-wrench shear zone, Rehamna Massif, Morocco

The Hercynian Rehamna Massif of Morocco is located in the Meseta belt, on the sheared boundary between the weakly strained Western Meseta Coastal Block and an intensely deformed Carboniferous basin. Deformation may be related to three superposed phases. The most important phases ( D 1 and D 2) result in a main composite S 1–2 foliation and are related to westward thrusting. D 4 is contemporaneous with the emplacement of syn- to late tectonic granitic plutons. Fold asymmetry and microstructures (rotated garnets, S/C tectonites) suggest a dominant, westward thrusting in eastern Rehamna. In central Rehamna, along the thrust zone front, stretching lineations are almost normal to the regional, coeval thrust displacement direction, and microstructures (rotated garnets, shear bands, asymmetrical pressure shadows) observed along the λ 1 direction (stretching lineation direction) suggest dextral, reverse southward wrenching. This particular strain pattern is interpreted as a product of thrust-wrench shearing, combining ductile thrusting and wrenching during the progressive, synmetamorphic shortening of the area, along the frontal tip of the Central Meseta thrust units.

Staurolite growth and metamorphic zones in the Kalak Nappe Complex of northeastern Porsangerhalvøya, north Norway

Four main thrust units have been differentiated in the Kalak Nappe Complex of northeastern Porsangerhalvøya. All exhibit Barrovian type prograde metamorphism. Although the metamorphic grade increases up the nappe pile the development of planar and non-kinematic fabrics is the same in relation to the deformation sequence in each of the thrust sheets. Early porphyroblast growth preserves a planar fabric developed during burial as a result of overthrusting. This was superseded by a randomly oriented and equidimensional non-kinematic fabric which developed by recrystallization, associated with increased fluid concentrations due to prograde metamorphic dehydration reactions. These fabric developments have been preserved in textures associated with the prograde alteration of staurolite and muscovite porphyroblasts. With increased fluid concentrations the shear strength of the rock declined and thrust propagation occurred; the associated shear stresses caused rotational fabrics in some garnets and aligned mineral growth.

Mesozoic volcanics of western sicily

The Mesozoic lavas and minor intrusions in the thrust sheets of western Sicily have the following characteristics: 1. The lavas in the Triassic Mufara Formation in the north were broken into fragments which rotated independently within the incompetent strata that enclose them. This behavior is characteristic of igneous rocks found within the more internal (northerly) thrust units. 2. The Jurassic lavas in the more external (southerly) units have consistent directions which agree with those of the Ammonitico Rosso limestones in the same zone and lie about 30° clockwise from those of coeval autochthonous formations in Tunisia.Schult's presumed Cretaceous directions from Custonaci on the north coast (similar to those found in the Cretaceous Scaglia Rossa at Terrasini to the east byChannel et al., 1980) are rotated still more (140°) with respect to those of the autochthonous Iblean platform of SE Sicily. These differences are believed to reflect rotation of the thrust sheets during tectonic transport in Cenozoic times, the internal units being the most strongly rotated. 3. All the igneous rocks are highly altered: generally the original mineralogy cannot be completely determined. Relative abundances of some of the less mobile elements (Ti, Sr, Y) suggest that they are intraplate basalts.

A review of Indian Phanerozoic palaeomagnetism: Implications for the India-Asia collision

An overview is presented of the Indian apparent polar wander path (APWP) for the Phanerozoic and in particular for post-Late Palaeozoic times. This APWP is compiled on basis of data available at October 1981 from peninsular and extrapeninsular Indo-Pakistan and from DSDP cores from the Indian plate. One of the more important and newly recognized features of this APWP is a large-scale Triassic-Jurassic loop. This loop indicates a changeover from a Late Palaeozoic-Early Mesozoic northwards and counter-clockwise rotational movement, with Greater India reaching moderately low southern latitudes, into a southwards and clockwise rotational movement during the Early to Middle Jurassic. Recognizable likewise in APWP's from other Gondwana continents, this loop reflects the opening of the Neotethys. Studies of extrapeninsular regions up to and north of the Indus-Tsangpo suture zone have shown wide-spread presence of magnetic overprints, which delineate two regionally confined age groups. Younger overprints (20–40 m.y.) predominate in the more external thrust zones. Older overprints (50–60 m.y.), in contrast, are found in the more internal zones both north and south of the Indus-Tsangpo suture zone. The latter are interpreted to reflect a late phase of relaxation in the Early Tertiary collision of Greater India with south-central Asia or off-shore island arcs, which occurred at equatorial to low northern palaeolatitudes (0°–10°N). Subsequent northwards movement over 2500–3000 km or more and impingement of Greater India into southern Asia resulted into large-scale underthrusting of Greater India along the Main Central Thrust beneath southern Tibet, and to clockwise rotation of thrust units in the Western Himalaya. A discrepancy between Indian palaeomagnetic data and results available todate from southern Tibet is discussed.

Inverted metamorphic zonation and large-scale thrusting in the Variscan Belt: an example in the French Massif Central

Summary Half of the northern part of the French Massif Central has been shown to be allochthonous. In the allochthonous rocks, preserved high-pressure granulites represent an early tectonometamorphic event which took place before thrusting. During the southward-directed overthrusting of this high-grade metamorphic slab a plurifacial inverted metamorphism was developed in the underlying sediments. Adjacent areas show different types of metamorphism in the autochthonous and parautochthonous pelites. The climax of the metamorphism is shown to have been reached by the end of the thrusting episode. This was followed by the retrogression of the high-grade rocks and the establishment of a low-pressure metamorphism in both the thrust unit and the autochthonous-parautochthonous terranes beneath. We describe this example of inverted metamorphic zonation associated with the large-scale thrusting of an already metamorphosed series.