Robert W. H. Butler

The terminology of structures in thrust belts

Abstract A review of structures and geometric relationships recognized in thrust belts is presented. A thrust is defined as any contractional fault, a corollary being that thrusts must cut up-section in their transport direction. ‘Flats’ are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ‘ramps’ are those portions of thrusts which cut across datum surfaces. Ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust. Lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike-slip movement. An array of thrusts which diverge in their transport direction may form by either of two propagation models. These are termed ‘piggy-back’ propagation, which is foreland-directed, and ‘overstep’ propagation which is opposed to the thrust transport direction. An array of thrust surfaces is termed an ‘imbricate stack’ and should these surfaces anastamose upwards a ‘duplex’ will result; the fault-bounded blocks are termed ‘horses’. A duplex is bounded by a higher, ‘roof’ thrust and a lower, ‘floor’ thrust. The intersection of any two thrust planes is termed a ‘branch line’. Thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut. A further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one. The movement of thrust sheets over corrugated surfaces, or the local development of thrust structures beneath, will fold higher thrust sheets. These folds are termed ‘culminations’ and their limbs are termed ‘culmination walls’. Accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust. These accommodation surfaces are termed ‘hangingwall detachments’ and they need not root down into the active thrust. This category of detachment includes dip-slip ‘hangingwall drop faults’ which are developed by differential uplift of duplex roofs, and ‘out-of-the-syncline’ thrusts which develop from overtightened fold hinges. Back thrusts, as well as forming as hangingwall detachments, may also form due to layer-parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp.

The tectonic history of Kohistan and its implications for Himalayan structure

The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan. The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.

Thrust tectonics, deep structure and crustal subduction in the Alps and Himalayas

The structural restoration of collision orogenic belts onto crustal templates provides important insights into the tectonic evolution, deep structure and amounts of plate convergence after the initial contact between two continental masses. Balanced cross-sections have been constructed, parallel to the local displacement directions, across the western Alps and western Himalayas and demonstrate very large amounts of crustal shortening above intra-crustal detachments. To achieve a balance, substantial volumes of lower crust must have been subducted beneath the two tectonic hinterlands. A model of eclogite metamorphism is invoked to facilitate this subduction and to explain the varying isostatic responses of the Alpine and Himalayan hinterlands. Patterns of eclogite metamorphism are controlled by the geometry of thrust profiles on a lithospheric scale: the development of crust-mantle detachments being of crucial importance. Such a profile is proposed for the Himalayas, suggesting that relatively small volumes of the footwall crust succumbed to eclogite metamorphism. In the Alps however, a steeper thrust profile apparently developed, emplacing mantle onto crust, causing wholescale eclogite metamorphism in the underlying Franco–Swiss crust. The resultant density increase would provide a mechanism of isostatic collapse and flexural subsidence in the Po plain region.

Structural styles and regional tectonic setting of the “Gela Nappe” and frontal part of the Maghrebian thrust belt in Sicily

The Gela Nappe of south central Sicily provides an example of a curved segment of an orogenic front that can be examined both onshore and offshore for deformational style and amount of shortening. Synorogenic sediments allow the deformation to be dated. Two distinct structural styles are observed in the Gela Nappe: The central salient part of the nappe (Caltanissetta basin) consists of a single thrust sheet containing a train of continuously tightening folds and the reentrant margins of the nappe (Sciacca and Monte Judica) consist of a stack of several thrust sheets. These different structural styles correspond to the pretectonic Mesozoic stratigraphy of the foreland plate. Carbonate platforms exist on the Adventure bank and Hyblean Plateau ahead of Sciacca and Monte Judica, respectively, while the Caltanissetta basin region appears to have accumulated basinal clay facies. Where the resistant carbonate stratigraphy provides a buttress to the propagation of the thrust front, deformation is taken up by imbrication on-steep ramps through the carbonates generating a relatively thick orogenic wedge. In the basinal setting, where no strong rheology exists, the low angle of friction on the clay detachment levels requires the growing thrust wedge to be much thinner with a very low foreland dip. Hence the thrust front propagates much farther forward into the basin than it does in the adjacent platformal areas, producing a nonlinear thrust front. In the basinal region, accretion of foreland material to the nappe by imbrication was only prominent during the Messinian when subaerial exposure prevented low-friction transport of the nappe across the highest levels of the stratigraphy. A steady thickening of the nappe by internal folding suggests an increase in friction along the basal detachment, possibly due to progressive compaction of the clays.

A structural analysis of the Moine Thrust Zone between Loch Eriboll and Foinaven, NW Scotland

Abstract The northern part of the Moine Thrust Zone as exposed around the valley of Srath Beag, Sutherland was developed by thrusts propagating in the tectonic transport direction. Deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time. The foreland has been progressively accreted onto the overriding Moine thrust sheet by duplex formation, a process which has continuously folded the roof thrust and the rocks above its hanging-wall. Fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging-wall a complex history of extensional and compressional strains normal to the transport direction. Folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography. Therefore there may be no correlation between structures across or along the thrust zone. This variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines.

The nature of ‘roof thrusts’ in the Moine Thrust Belt, NW Scotland: implications for the structural evolution of thrust belts

Buried thrust systems, commonly analysed in terms of duplex structures, are often interpreted as forming in a particular sequence. This model considers that only one thrust is active at a time and the resulting structural geometry is well ordered. However, a reappraisal of structural geometries from the Moine Thrust Belt in NW Scotland has shown that many apparent duplex structures are more complex. Although the upper (roof) thrust of some duplexes may be folded and bulged by underlying imbricate slices (in accordance with simple duplex models), the footwall to these roof thrusts cut up and down stratigraphic section (in conflict with the simple duplex models). Examples include the Glencoul Thrust with its underlying imbricate system, and the Foinaven ‘duplex’. Simultaneous slip on an array of imbricate thrusts can bulge duplex roofs (relationships classically used to infer piggy-back sequences of thrust ‘propagation’). However, activity on more hinterland-ward imbricate thrusts can cause the roof onto which they branch to truncate more foreland-ward structures. In this fashion, the ‘duplex roof’ can cut up and down stratigraphic section, mimicking an extensional fault. By varying thrust activity in three dimensions within an imbricate system, local parts of ‘roof thrusts’ can generate domains of overstep behaviour that have only local significance. Existing interpretations of overstep relationships in the Moine Thrust Belt as representing late-orogenic low-angle extensional faults may need revision. The new interpretations of deformation activity, where displacements in thrust arrays happen synchronously rather than sequentially, are consistent with emergent thrust and fold belts, with analogue experiments and with mechanical models of orogenic wedges.

Anatomy of a continental subduction zone: The main mantle thrust in Northern Pakistan

ZusammenfassungDer Himalaya bildet ein ideales, natrliches Laboratorium für Untersuchungen von Deformationsprozessen in kontinentalen Krustengesteinen während der Kollision bzw. Orogenese. Hier werden neue Daten vorgelegt, die sich mit der strukturellen Entwicklung der Hauptmantelüberschiebung im Himalaya von Nordpakistan im Gebiet um den Nanga Parbat befassen. Die Hangendeinheiten oberhalb der Störung liegen in einem relativ hohen Niveau innerhalb des »Kohistan arc terrane«, das auf die indischen Kontinentalgesteine überschoben wurde. Diese Überschiebung entstand wahrscheinlich als Rücküberschiebungsstruktur im Hangenden der Subduktionszone vor der Kollision. Im Hangenden befindet sich eine ca. 1 km breite Scherzone, die sich unter amphibolitfaziellen Bedingungen gebildet hat. Die durch »simple shear« erzeugten Deformationen sind mit ihren neuen Gefügen parallel zur Hauptüberschiebungszone ausgerichtet. Die Gefüge wurden nachfolgend von Extensionsund Kompressionsbewegungen im Bereich von ca. 100 m um den Überschiebungskontakt erneut unter amphibolitfaziellen Bedingungen erfaßt und deformiert. Das Liegende der Hauptüberschiebung besteht aus einem alten Basementkomplex (den Nanga Parbat Gneisen), die von deutlich abgesetzten, amphibolitfaziellen Metasedimenten überlagert werden. Diese Sedimenthülle bestehend aus Psammiten, Peliten und Marmoren mit lokalen Metabasiten stößt entlang der MMT direkt gegen die Gesteine des Kohistanbogens. Das Fehlen von Strukturen, die auf gleichbleibende rheologische Unterschiede hinweisen würde, läßt vermuten, daß der Großteil der in ihnen enthaltenen Deformationsgefüge auf einmal während beträchtlicher tektonischer Auflast entstanden ist. Vorher wurden die indischen Hüllgesteine »passiv« unter den Kohistanbogen bis in den Bereich der Amphibolitfazies subduziert. Die Folgerungen aus der sich über die Zeit entwickelnden Breite dieser Scherzone werden diskutiert und die Bedeutung für die Vorhersage der Charakteristik von mitteltiefen krustalen Scherzonen, insbesondere in Verbindung mit seismischen Reflektionsprofilen betont.AbstractThe Himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny. New information is presented concerning the structural evolution of the Main Mantle thrust zone in the Himalayas of N Pakistan, in the region around Nanga Parbat. The hanging-wall lies at relatively high levels within the Kohistan arc terrane which has been emplaced onto Indian continental rocks. This thrust probably originated as a breakback structure in the hanging-wall to the pre-collisional (oceanic) subduction zone. The present hanging-wall contains a shear zone of about 1 km width developed under amphibolite facies conditions. Simple shear dominant strains have developed new fabrics parallel to the main thrust zone. However, these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact, again developed under amphibolite facies conditions. The footwall consists of an old basement complex (the Nanga Parbat gneisses) overlain by a distinct suite of metasediments now at amphibolite facies. This cover assemblage of psammites, pelites and marbles with local metabasites consistently lies directly against rocks derived from the Kohistan arc along the MMT. The absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens. Prior to this the Indian cover rocks will have been »passively« subducted beneath the Kohistan arc until into amphibolite facies. We discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid-deep crustal shear zones, particularly from seismic reflection profiles.RésuméLHimalaya constitue un laboratoire naturel idéal pour létude des processus de déformation de la croûte continentale au cours dune orogenèse de collision. Les auteurs présentent des informations nouvelles relatives à lévolution structurale de la zone du Main Mantle Thrust dans la région du Nanga Parbat au nord du Pakistan. Le toit de cet accident occupe un niveau assez élevé dans le «Kohistan arc Terrane» qui a été charrié sur les roches du continent indien. Le charriage doit probablement son origine à une structure en retour apparue au-dessus de la zone de subduction pré-collisionnelle (océanique). Le toit actuel de laccident contient une zone de glissement (shear zone) épaisse denviron 1 km et formée dans les conditions du facies des amphibolites. Les déformations engendrées par glissement simple (simple shear) ont développé de nouvelles fabriques parallèles à la surface de charriage. Cependant, dans une tranche dune centaine de mètres à partir du contact du charriage, ces structures ont été reprises par des cisaillements extensionnels ou compressionnels, toujours dans les conditions du facies des amphibolites.Le mur de laccident est formé dun complexe ancien (le gneiss du Nanga Parbat) surmonté dune série de métasédiments distincts qui présentent aujourdhui le facies des amphibolites. Cette couverture de psammites, de pélites, de marbres et de métabasites locales est directement en contact le long du MMT avec larc du Kohistan. Labsence, dans cette couverture, de structures témoignant de contrastes rhéologiques marqués, suggère que la grande majorité des structures déformatives ny ont été développées quaprès un enfouissement tectonique important. Avant cela, les roches de la couverture indienne ont dû être subductées passivement sous larc du Kohistan, jusquau facies des amphibolites. Les auteurs discutent ces conclusions en termes dévolution temporelle dune shear zone et en commentant les implications dans le domaine de la prédiction du caractère des shear zones de profondeur crustale moyenne, en particulier à partir des profils de sismique réflexion.Краткое содержаниеКак известно, Гималаи являются идеальной естественной лабораторией для изу чения процессов дефо рмации пород материковой ко ры во время коллизии плит, или процессов ор огенеза. Приведены ре зультаты последних исследова ний о развитии структ ур надвига основного по крова Гималаев в севе рной части Пакистана на На нда-Парбат. Стратигра фические единицы кровли над ра зломом залегают на ср авнительно высоком уровне в “Kohistan arc terrane” массиве Когистанской дуги —, н адвинувшись на матер иковые породы индийского по луострова. Этот надвиг образовался, в ероятно, как структур а возвратного надвига в кровле зоны засасывания еще до коллизии плит. В кровл е находят зону скола п римерно в 1 км шириной, которая о бразовалась в услови ях амфиболитной фации. Т екстура этих деформа ций, вызванных простым сд вигом, простирается п араллельно главной линии надвиг а. При последующих деформациях растяже ния и сдавления в реги оне примерно 100 м над контак том с надвигом тексту ра пород оказалась снов а измененной до фации амфиболита. Подошва основного на двига состоит из древнего цоколя — гне йсов Нанга-Парбат —, ко торый явно отличается от по крывающих его метасе диментов амфиболитной фации. Э тот осадочный чехол, составленный из псам митов, пелитов и мрамо ра с отдельными метабазитами, натолк нулся вдоль ММТ на породы дуги Когист ан. Отсутствие структ урных единий, характерных д ля реологических раз личий, не подвергшихся изме нениям, разрешает пре дполагать, что большая часть сод ержащихся в них дефор мационных структур, образовала сь одновременно и одноразово в результ ате значительной тек тонической нагрузки. До этого соб ытия породы индийско го покрова засасывалис ь пассивно под дугу Ко гистана. Обсуждаются последс твия образования это й широкой зоны скола и значение полученных результа тов для предсказания хар актера зон скола в сре дних горизонтах коры и возможности вы явления этого скола с помощью сейсмическ их методов.

‘Structural evolution in the Moine of northwest Scotland: a Caledonian linked thrust system?’

A model is proposed whereby the Caledonian metamorphic basement-cover complex of northwest Scotland (the Moine) is considered as a linked thrust system. This system lies between the Moine thrust at its base and the Naver–Sgurr Beag slide at its top. Ductile fold and thrust zones, which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies, are interpreted as decoupling from a detachment presently situated at relatively shallow depths. This model is illustrated by two preliminary balanced cross-sections. These imply shortening across the northwest Scottish Caledonides in excess of 130 km and probably over 200 km. When these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth. The most likely location for the lower crustal wedge is beneath the Grampian Highlands.

Crustal Scale Thrusting and Continental Subduction During Himalayan Collision Tectonics on the NW Indian Plate

Following the subduction of Tethyan oceanic lithosphere beneath Asia and Kohistan the continued convergence between the upper plate and the Indian continent led to thrust stacking of Indian crust to form the Himalayas. This lasted from Oligocene to Recent times and in an attempt to evaluate displacements, a series of balanced cross-sections have been constructed across the belt. In Pakistan these illustrate that over 600km of relative convergence between India and the Kohistan complex north of the Eocene suture zone has occurred by SSE-directed thrusting. This deformation only involves Indian upper crust at present outcrop levels so that the lower crust and remaining lithosphere must have been subducted beneath Kohistan and Tibet. The northern edge of the Indian lower crust may lie beneath the Pamirs. Similarly large amounts of shortening (several hundred kilometres) are implied by other balanced crustal sections through the central Himalayas and western Pakistan. The continuity of thrust systems around the NW margin of the Indian continent is proposed so that thrusts which stack continental crust step off into oceanic lithosphere in the west. This thrusting mechanism accounts for a substantial fraction of the total, 1200–2000km relative convergence between stable India and Asia. Further shortening in the Tibetan region which developed after the Eocene continent-continent collision must be added to the displacements on thrusts which stack Indian lithosphere. Deformation within the entire collision zone approximates more closely to an essentially vertical plane strain model rather than to a process of lateral expulsion of Tibet towards the east.

Structural evolution of the Moine thrust belt between Loch More and Glendhu, Sutherland

Synopsis Reinvestigation and new mapping confirm that the area contains a number of large thrust sheets and imbricate slices of Cambrian cover and Lewisian basement but emphasise that they are bounded by an interconnecting network of WNW-directed faults. The network demonstrably propagated, piggy-back fashion, into the foreland since higher structures are cut or bulged up by lower imbricates. These new interpretations require a revision of structural units in the Glendhu area. The margins of culminations, which resulted from the continual stacking of imbricate slices, are the sites of extensional faulting which locally post-date thrust structures. However, some of the major thrusts in the area have primary extensional geometries in their footwalls and hanging walls since they cut down stratigraphic section towards the foreland. The proposed explanation is that bedding in the foreland Cambrian shelf was dipping gently to the SE, possibly in response to isostatic loading by the emplacement of the c. 10 km thick Moine thrust sheet. Datum-parallel thrust flats would then cut down stratigraphic section with an extensional sense. Early extensional strains which locally predate imbricate thrusts in those Cambrian quartzites in the immediate footwall to the Moine thrust may reflect a major gravity spreading component in the initial phases of the emplacement of the Moine sheet. The geometry of thrusts in the Glendhu culmination is illustrated on a balanced cross-section. The hanging wall of the Glencoul thrust, which includes a stack of interthrust Cambrian cover and Lewisian basement termed the Aisinnin imbricates, restores to a width of over 15 km. This can be added to the displacement on the Glencoul and lower thrusts as estimated from the offset of foreland structures, of 25–33 km, to give a total restoration for the footwall of the Moine thrust of at least 40 km.