M. P. Coward

Preliminary conclusions of the Royal Society and Academia Sinica 1985 geotraverse of Tibet

The 1985 Chinese/British expedition to the Tibetan Plateau attempted to solve the question of the origin of the very thick crustal rocks in this region. Continuing northwards movement of the Indian plate over the past 38 Myr has given rise to severe folding and thrust faulting, causing crustal thickening by internal deformation. Previous collisions of microplate terranes derived from Gondwanaland occurred during Mesozoic times but the Kun Lun terrane of northern Tibet was already part of Laurasia by the Carboniferous.

Alpine tectonics — an overview

Summary This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume. The Alpine Mesozoic ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of Africa towards the east relative to a fixed Europe. The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the Cretaceous, as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive Iberia and the southern part of the European plate to the E. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-Alpine nappes of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the Alps in Switzerland and Austria is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory. Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.

Collision tectonics in the NW Himalayas

Summary West Himalayan tectonics involve the collision of microplates between the Indian and Asian Plates. The Kohistan Complex consists largely of tightly folded basic volcanics and sediments generated as Late Jurassic to Late Cretaceous island arcs. These were intruded by post-folding Mid-Cretaceous — Eocene plutonics produced from continued subduction of the Indian Plate after closure of a suture between Kohistan and the Karakorum. The Himalayan structures show major thrust sheets and the Kohistan Arc is essentially a crustal ‘pop-up’ with southward-upright and northward-verging structures developed above a thick ductile decoupling zone (the Indus Suture), which can be traced for >100 km beneath Kohistan on large reentrants. This pop-up formed by a two stage process, closure of the Northern Suture followed by closure of the southern Indus Suture. Granitic rocks of the Kohistan-Ladakh Batholith (dated at ≅ 100-40 Ma) post-date most of the structures related to the Northern Suture but were deformed and carried southwards on shear structures related to the Indus Suture. Post-collisional deformation carried this Kohistan Complex on deep decoupling zones over the Indian Plate on a series of imbricated gneiss sheets, the thrusts climbing up section in the movement direction so that in the far S some override their own molasse debris. Folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms—i.e. the Nanga Parbat and Hazara Syntaxes. The Nanga Parbat Syntaxis probably formed due to a shear couple near a branch line where one of the main Himalayan thrusts joined the Indus Suture beneath Kohistan. Crustal delamination, to produce the imbricated gneiss sheets, could not account for all the displacement of India into Asia, suggested by palaeomagnetic data. There must also have been lateral displacement as demonstrated by the large oblique-slip shear zone in the Hunza Valley, N of Kohistan.

Complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones

Abstract Many of the complex strain patterns seen in shear zones and thrust zones, such as variable fabric orientations, refolded folds and fabrics, together with folds with hinges almost parallel to the main transport direction, can be explained in terms of differential movement within the shear zones. These strains are developed at the frontal and lateral tips of the zones as they propagate. Examples are taken from the Moine thrust zone of Scotland which show variations in strains particularly at the lateral tips. The form of differential movement described here may lead to complex strain paths and non-plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts.

Structural inversion and its controls : examples from the Alpine foreland and the French Alps

AbstractPositive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form obliq...

The structure of the 1985 Tibet Geotraverse, Lhasa to Golmud

The structures of Tibet were generated during the accretion on to the Asian plate, firstly of the Qiangtang Terrane during the Triassic, then the Lhasa Terrane during the Jurassic -Cretaceous and finally the Indian continent during the Palaeogene. The southern Kunlun mountains show intense deformation associated with the accretion of deep water sediments on to an active plate margin .The deformation was essentially by footwall propagation of thrusts, though there was pronounced out-of sequence thrusting with the deformation of basins above the main thrust zone, and the back steepening and back thrusting of earlier structures. The Jinsha Suture probably represents the southern edge of this zone. The Banggong Suture between the Qiangtang and Lhasa Terranes is characterized by pre-collisional ophiolite obduction for over 100 km to the south across the Lhasa T errane, plus local intense intracratonic deformation of parts of the Lhasa Terrane. However, for this collision there is now very little evidence for intense deformation along the line of the suture and the Qiangtang Terrane itself remained only weakly deformed throughout. Post—Middle Cretaceous, pre-Tertiary deformation of the Lhasa region produced upright- to north-verging folds which decrease in in tensity north wards. They may have been formed at the margin of the Gangdise batholith, or they may have originated from early collisional phases along the line of the Indus-Zangbo Suture. However this deformation is approximately synchronous with the more intense deformation of the Xigatse flysch on the accretionary prism and is therefore probably subduction-related, predating collision. Tertiary deformation is relatively widespread across Tibet, producing SSE-directed thrusts across the Fenghuo Shan region of the Qiangtang Terrane and across the northern part of the Lhasa Terrane. Several hundred kilometres shortening can be estimated to have occurred during this deformation, probably reworking older Mesozoic structures. How ever this shortening is insufficient to provide all of that estimated from palaeo magnetic work or from a study of displacement rates of the Indian plate, and much of the displacement of India into Asia during the Tertiary must be taken up on strike-slip faults in Tibet or on thrusts an d strike-slip faults in central Asia north o the Tibetan Plateau. The Tertiary shortening cannot account for all the thickening o f the Tibetan crust.

Strain within thrust sheets

Summary The finite strains within the thrust sheets of the Moine Thrust Zone in NW Scotland have been factorized into components of simple shear and longitudinal strain. Variations in these strains have been examined in the Cambrian sediments in the Eriboll and Assynt areas and strain maps produced of the Glencoul Nappe, Assynt. There are heterogeneous simple shear strains parallel to the layering resulting from bending of the nappe over a step in the thrust plane and to frictional drag at the base of the thrust. Layer parallel shortening and associated layer parallel thickening occur in the front of the thrust zone at Eriboll, above a decoupling plane in the lower part of the Cambrian sequence. Within the Cambrian rocks of the Glencoul Nappe in Assynt, there is a steady eastward increase in intensity of layer parallel shortening togeather with local anomalous zones characterized by more intense shortening or extension. These anomalous zones overlie shear zones in the Lewisian basement. There are variations in shear strain on the plane which contains the normal to bedding resulting from differential movement of the nappe. These shear strains distort and fold the bedding planes. Combinations of these shear strains and the longitudinal strains result in a wide range of ellipsoid shapes from oblate to prolate and also explain the pattern and sequences of folds in the nappes and mylonite belts. The folds are formed by local variation in shear and longitudinal stress and, hence, there may be no simple correlation of fold phases along, or across, a thrust zone.

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.

FRONTAL PART OF THE NORTHERN APENNINES FOLD AND THRUST BELT IN THE ROMAGNA-MARCHE AREA (ITALY) : SHALLOW AND DEEP STRUCTURAL STYLES

In this study, surface geological data resulting from a detailed field survey, including structural and biostratigraphic analysis, have been integrated with subsurface (seismic lines and well logs) data in order to reconstruct the tectonic evolution of the external zones of the northern Italian Apennines in the Romagna-Marche foothills and Adriatic Sea areas. This integrated analysis shows: (1) a late Messinian to lower Pleistocene progression of structural development from the hinterland to the foreland of the studied sector of the thrust belt; (2) relatively limited (≤20%), southward increasing, amounts of shortening (obtained by the construction of line-length balanced and restored geological cross sections); (3) a regional deformation style characterized by the presence of backthrusts associated with most foreland-vergent thrust ramps, leading to quasi-symmetric uplift and a low critical taper for the wedge, typical of foreland fold and thrust belts with a weak basal decollement (Triassic anhydrites in the present case); (4) an important influence of basement faulting which, despite a general basement-cover decoupling, appears to control stress localization in the latter, producing linkage of basement and cover stuctures in a combination of thin- and thick-skinned tectonic styles; and (5) contrasting structural styles characterizing deep features, as imaged by seismic reflection profiles, and shallow ones. Deep stuctures consist of growth anticlines bounded by major thrust ramps and back limb back thrusts, separated by broader, open synclines, both involving a Mesozoic-Paleogene, mainly carbonate, passive margin succession. In the crestal zones of major anticlines, shallow structures, affecting Neogene terrigenous foredeep sediments, show a complex pattern of upright to recumbent folds (of tens to hundreds of meters wavelength) related to minor thrusts and backthrusts. Deformation of the Mesozoic-Paleogene multilayer appears to be dominated by thrust propagation in the cores of early formed anticlines developed by buckling instabilities. The overlying Neogene deposits are detached from the carbonate substratum along the base of the foredeep succession. Bedding-parallel slip occurring along this detachment level appears to be accommodated by the complex structures in the crests of major anticlines, where the thrusts lramp to the surface cutting up section. Complex shallow structures, interpreted to accommodate at shallow structural levels the deep deformation, would therefore represent a geometrical requirement for maintaining strain compatibility across the shallow detachment level located at the base of the foredeep succession.

Strain within ductile shear zones

Abstract The intensity of strain has been measured from the change in shape of clusters of mafic and felsic minerals within a ductile shear zone of short finite length from Botswana and a much longer planar shear zone from North Uist, Scotland. The strain measurements from North Uist have been supported by measurements of the anisotropy of magnetic susceptibility. These measurements indicate that simple-shear movement was accompanied by pure shear across the zone. A model is proposed for the strain during the growth and propagation of shear zones. Shear zones with a high rate of shear strain relative to rate of propagation may have a sigmoidal form. Such shear zones may coalesce to enclose lozenge-shaped blocks of less deformed rocks which carry a tectonic fabric at a high angle to the actual shear zone.