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Introduction: the deformation of continental crust and Mike Coward's impact on its understanding

R. W. H. BUTLER 1, R. H. GRAHAM 2 & A. C. RIES 3 1Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK (e-mail: butler@earth, leeds, ac. uk) 2Amerada Hess, The Adelphi Building, 1-11 John Adam Street, London WC2N 6AG, UK 3Ries-Coward Associates Ltd, 70 Grosvenor Road, Caversham, Reading RG4 5ES, UK

Understanding geological structure and struc- the roots to Proterozoic orogens. These tural evolution requires imagination, and the deformed gneiss terrains formed obvious places ability to see simple patterns in complex data to test and develop quantitative methods of and make the simplicity evident. It requires the strain and kinematic analysis, especially applying transfer of insight and approach from one area of the methods of Ramsay (1967). Later the shear discipline (geographical or scientific) to another. zone model of Ramsay & Graham (1970) was To Mike Coward it was second nature to do these used to understand the kinematic evolution of things, and he stood out as one of the most inno- these regions, and by inference, the deep conti- vative structural geologists of recent years. He nental crust. Escher & Watterson (1974) were the was a great exponent of what is currently referred first to upscale the model to understand crustal- to as 'up-scaling': understanding the significance scale deformation, using the Nagssugtoquidian of small-scale, local observations and datasets structures in southern Greenland. Coward used then using them to elucidate geological evolution the model qualitatively, first to explain the 3D on a crustal scale. nature of crustal-scale shear zones and networks This introductory paper is a brief review of of shear zones that define areas of deformed and some key themes in the study of the deformation less-deformed crust of the Lewisian (reviewed of the continental crust, and how Coward influ- elsewhere in this volume), then more widely and enced them. It focuses particularly on thinking at continental scale in, for example, the Limpopo in structural geology during the past 50 years or mobile belt and adjacent Archaean cratons, and so, but does not cover such topics as seismicity, the Damaran and Zambezi belts. Rather than geodetic data or microstructural approaches attempting to understand ductile deformation in deformation studies. The emphasis is on the in terms of correlating fold phases and tightly importance of understanding medium- to large- defined sequences of deformation, he applied scale structural geometry. This was Mike models from linked fault systems. He recognized Coward's fort6, for it emphasizes the key role of that there is a continuum from ductile shear at field geology. The discipline of field mapping is a deep crustal levels to thrusting or extension at great aid in the interpretation of 3D relationships higher levels. This was clearly illustrated in a in seismic data volumes. It is no coincidence number of Coward's papers through the 1970s that this account begins in the field but leads into and 1980s (e.g. Coward 1984) and, although self seismic data. It is a journey followed not only by evident now, they represented the cutting edge Mike Coward but also by many of the authors thinking at the time. contributing to this volume. Thrust belts Crustal-scale shear zone models The external parts of mountain belts are com- Field geologists have long been aware that monly marked by zones of thrusting and tectonic structures seen in outcrops of high-grade meta- imbrication. Many also contain basement slices morphic rocks may be used as analogues for the of variable thickness and deformation state. deformation in situ in the deep crust. Argand Pioneering research in the Moine thrust belt (1924) used structural styles seen in the Alps in the 1880s by Peach et al. (1907) and their con- to support his models of crustal deformation temporaries was revisited by Elliott & Johnson beneath the Himalayas and Tibet. Sutton & (1980). They employed an approach that Watson (1951) recognized that tracts of was influenced by trans-Atlantic ideas on deformed gneisses in the Lewisian represented thin-skinned tectonics and section balancing

From: RIES, A. C., BUTLER, R. W. H. & GRAHAM, R. H. (eds) 2007. Deformation of the Continental Crust: The Legacy of Mike Coward. Geological Society, London, Special Publications, 272, 1-8. 0305-8719107l$15 © The Geological Society of London 2007. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

2 R.W.H. BUTLER ET AL. developed in the external zones of the Rocky belt, to propose that the Moine thrust followed Mountains and Appalachians. Coward realized a shallow trajectory in the upper crust and that the importance of the approach but questioned the NW Highlands were underlain by Lewisian many of the strict 'thrust belt rules'. He devel- basement of the Caledonian foreland. He went oped a new understanding of thrust sequences on to apply these methods to the Himalayas that required more complex structural evolution (Coward & Butler 1985; Butler & Coward 1989), than previously envisaged and there was a new well ahead of the acquisition of the crustal emphasis on the interpretation of strain patterns geophysics datasets by the INDEPTH team (e.g. in thrust sheets (chiefly reviewed by Coward et al. Zhao & Nelson 1993). In essence these researches 1992). In the late 1970s accepted wisdom related were a natural continuation from looking at strain patterns to mechanisms of thrust sheet deformed gneiss terrains but show the impor- emplacement, such as gravity gliding and gravity tance of up-scaling field observations in the spreading (e.g. Elliott 1976). Coward and his interpretation of deep seismic reflection profiles. students (e.g. Coward & Kim 1981; Fischer & Subsequently, these approaches have been Coward 1982) introduced a matrix approach to followed with modifications, closely associated describe the 3D finite strain state using data from with deep seismic acquisition programmes, such deformed Skolithos in the Cambrian Pipe Rock as the NRP20 project in the Swiss Alps (Pfiffner Formation. They showed that the strains related et al. 1997) and CROP in the Italian Apennines to distributed shearing that in turn reflected par- (Scrocca et al. 2003). titioning of deformation away from simple thrust displacements. The benchmark description of 3D strain patterns by Coward & Potts (1983) Orogenic systems: Himalayas-Tibet provided a framework for many of the general One of the key advantages of studying modern strain studies associated with transpression or orogenic systems is the precision gained from transtension (e.g. Fossen & Tikoff 1998). The key geochronological and stratigraphic studies. In to these strain papers in the Moine thrust belt lay this regard, the Himalayan-Tibetan system has in relating the outcrop-scale deformation to the been a fundamental test track for tectonic larger-scale geometric evolution of the system, models. Research in this area received a great especially to possible displacement gradients that impetus in the mid-1970s with the opening of the originated from thrust zone propagation. Karakoram Highway and consequently much of Fieldwork in the north Assynt district the early work focused on the NW Himalayas of revealed a series of structural relationships that Pakistan, building on the recognition that much Coward (1982) interpreted as reflecting exten- of Kohistan represented a Cretaceous island arc, sional faulting within the thrust belt. His map- caught up in the collision zone (Tahirkheli & Jan ping showed that these faults linked onto thrusts, 1979). Structural studies in the exhumed lower indicating that they were associated with thrust arc (e.g. Coward et al. 1987) provided the plat- sheet translation. His surge zone hypothesis, form for subsequent research, establishing the although controversial, formed the basis of section as an important outcrop analogue for larger-scale geodynamic interpretations of thrus- lower crustal processes (e.g. Treloar et al. 1990). ting in NW Scotland (e.g. Coward 1983) that There are also seductive similarities in the continue in some analyses today (Holdsworth patterns of strain localization and kinematics to et al. 2007). basement gneiss terrains, such as parts of the Lewisian complex, that Coward was not slow to Crustal-scale thrusting models recognize. An important finding from the research in The research in the Moine thrust belt coincided NW Pakistan was the recognition that the Indian with imaging the subsurface continuity of thrusts continental crust beneath the Kohistan arc had in the Appalachians (Cook et al. 1979) and the experienced subhorizontal extension and thrust- early applications of deep seismic reflection ing. Coward et al. (1987) informally termed soundings in Britain (the BIRPS programme; e.g. this 'hedgehog tectonics', famously likening Brewer & Smythe 1984). Coward was not alone the process to that of road-kill, harking back to in understanding the importance of deep seismic Termier's (1904) 'crushing sledge' model for data. Soper & Barber (1982) were the first to Alpine orogenesis. This kinematic relationship is apply structural restoration concepts to crustal- now embraced by the protagonists of 'channel scale interpretations of the Moine thrust. They flow' (e.g. Beaumont et al. 2001). proposed a crustal duplex model, with steeply Although suggested by seismicity and remote rooting thrust trajectories. However, Coward sensing, the fundamental differences between the (1980; Butler & Coward 1984) used field data, tectonics of Tibet and the Himalayas were firmly which indicated large displacements in the thrust established only after the fieldwork conducted Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 3 on the 1985 Tibetan Geotraverse expedition recognized in the region (e.g. Coward & Dietrich (Coward et al. 1988). Although there are struc- 1989). However, the lessons have barely been tures that indicate subhorizontal shortening, applied to the restacked continental margin in in general these date from early in the history of the Himalayas, notwithstanding many attempts collision tectonics in the region. The younger at crustal-scale thrust interpretations (discussed structures are largely strike-slip and normal above). In this light, the existing interpretations faults. There has been much refinement of these of crustal structure are likely to be highly findings in the past 20 years, especially linking non-unique. the active surface deformation detected from geodetic data (e.g. Zhang et al. 2004) to seismo- Basin evolution logical results (e.g. Ozacar & Zandt 2004). These results illustrate the importance of differ- The late 1980s saw a shift in the focus of struc- ent strain states down through the crust. The tural geology as commercial seismic reflection importance of low-angle thrusting within the data became routinely available for academic Himalayas, suggested by Argand (1924) and research. Coward was quick to recognize the developed by Coward & Butler (1985) and others opportunities for fundamental structural since (DeCelles et al. 2001), is apparently research and essentially reframed his paper on confirmed by modern seismological studies crustal deformation styles for contraction (Schulte-Pelkum et al. 2005). (Coward 1983) in terms of extensional tectonics (Coward 1986), Like others (e.g. Watson 1985), Inversion he realized that many of the basin structures around the British Isles owed their trends to Models for thrust system evolution, derived and earlier structures. Thus Enfield & Coward (1987) modified from those developed in the Rocky showed that the West Orkney basin preferen- Mountains, were applied to NW Scotland (e.g. tially reactivated the Caledonian deformation Elliott & Johnson 1980) and to Tethyan orogens, fabric, as imaged in the MOIST deep seismic data such as the Alps (e.g. Boyer & Elliott 1982). (e.g. Brewer & Smythe 1984). This was not simply However, these applications were not especially reactivation of pre-existing faults. Rather it was successful because the pre-orogenic stratigraphic the penetrative anisotropy that controlled the template was not simple. The Mesozoic inherit- orientation of normal faults. Coward's work on ance in the western Alps was brought vividly into the Caledonian controls of rifting, off the north modern context by Lemoine et al. (1986), who coast of Scotland and in Orkney, led to a wider described tilted fault blocks and half-graben interest in the influence of Caledonian structures analogous to those on present-day continental in the Mesozoic rifting around Britain. Much margins. Some of these are barely deformed but of this was reviewed in a series of contributions simply elevated. Others, as recognized by Gidon to the Millennium Atlas (Coward et al. 2003; (1981), were significantly shortened during Zanella & Coward 2003; Zanella et al. 2003). Cenozoic mountain building. Coward took these These basement-reactivation ideas have been models and developed them, initially in the Alps developed and expanded by several groups (Gillcrist et al. 1987) and then more generally (reviewed, for example, by Butler et al. 1997). (e.g. Coward 1994). His reappraisal of thrust Halokinesis is commonly a key factor in the geometry in the Kirthar Range of Pakistan shows structural evolution of sedimentary basins, how a 'thin-skinned' interpretation (e.g. Banks including the southern North Sea. Classically salt & Warburton 1986) requires very large (and very movement in intracratonic basins, such as the unlikely) net displacements. Inverted normal North Sea, is viewed in vertical terms alone so faults, without much displacement, are a more that structures are expressed solely in terms realistic alternative. These types of reappraisal of salt pillars, walls and diapirs (reviewed by were extended to other settings, such as the Jackson & Talbot 1994). However, work on Apennines (Coward et al. 1999a), where they passive continental margins, such as the northern continue to have an impact on the understanding Gulf of Mexico (e.g. Humphris 1978; Wu et al. of orogenic belts (e.g. Tavarnelli et al. 2004). 1990), showed that salt migration can be strongly Clearly, in the orogens of the western Tethys, asymmetrical, leading to down-slope migration where pre-existing basin-controlling faults can be both of the salt and the overlying sedimentary readily inferred from the lateral variations in prism. Stewart & Coward (1995) were amongst Mesozoic stratigraphy (e.g. Bosellini 2004), some the first to show that similar asymmetrical form of structural inheritance is likely. This gen- processes also occur, albeit on a smaller scale, on eral recognition is well established (e.g. Jackson extensional tilt blocks within the southern North 1980). The implications for crustal balancing are Sea. 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4 R.W.H. BUTLER ET AL.

Basic structural concepts have long been little-known Torridon area. Detailed field studies important in the exploration for natural are used to assess the applicability of ideal simple resources and there has been significant exchange shear zone models in describing kinematics on a of ideas between the research and industrial variety of scales. Although ideal simple shear sectors. Famously, the impetus for the re- may explain local structures, it is less applicable examination of many thrust belts, especially in to broader zones of deformation and on the scale Europe (e.g. Boyer & Elliott 1982), came from oil of the crust. The tradition of detailed kinematic exploration in the Canadian Rocky Mountains studies in the Lewisian is also continued by (Bally et al. 1966). Although it has much older Tatham & Casey, who re-examine the Badcall origins, section balancing, developed in the shear zone, described by Coward & Potts (1983). Rockies (Dahlstrom 1969), was applied first to The paper accepts the geometric challenge of the Moine thrust belt (Elliott & Johnson 1980) deciphering spatially varying lineation patterns, and then to the Pyrenees (e.g. Williams & Fischer invoking variable shear directions rather than 1984), the Alps (Beach 1981) and beyond. heterogeneous combinations of pure shear and Coward's research forms part of this tradition, simple shear. Similar scaling issues are addressed especially in the use of regional-scale tectonic by Cosgrove, who examines shear zone develop- understanding to inform knowledge of specific ment in a brittle-ductile setting. Here the peren- development prospects. As hydrocarbon explo- nial problem of relating finite structures to ration reaches out into regions of increasing tech- deduce the orientation of regional stress axes nological challenge and geological complexity, from outcrop data is addressed. the demands on structural geology continue. The next group of papers retains a focus on Similar challenges face the minerals sector. In a structural geometry and draws on examples from typical piece of innovation towards the end of his the Caledonian thrust and shear systems of NW career, Coward took the oil industry's approach Scotland. As with the papers on Lewisian kine- to basin evolution into mining through his matics and structure, Alsop & Holdsworth build consultancy in gold exploration (Coward et al. on the pioneering strain models of Coward & 1995). Such applications remain in their infancy; Potts (1983). They examine fold patterns in the indeed, they are highly controversial and mylonites of the Moine thrust belt, drawing challenge existing thinking. comparisons with other 3D folding systems, such as slumps. The patterns of folding are used to About this volume deduce zones of differential movement in ductile thrust complexes. Coward's work on thrust sys- The papers in this volume all build on Coward's tems in the NW Highlands in the 1980s continues legacy. Some offer distinctly personal com- to influence research on the structural evolution mentaries and reviews. Others present the results in the region together with the outer parts of of structural studies from settings that he mountain belts in general. Butler et ai. examine researched extensively to those that he never the southern part of the Moine thrust belt, visited but that nevertheless follow the tradition. an area long overlooked in favour of the better They are arranged so as to track, in broadly chro- known Assynt and Eriboll districts. They nological order, the research themes that Coward describe the structural relationships between developed through his career. relatively minor imbricate thrust systems and Coward's early work in the Precambrian the major thrusts. A key conclusion is that end- basement of NW Scotland is summarized by member thrust-sequence models are not appli- Graham. This paper outlines his structural cable in detail and that displacements in thrust studies in the Lewisian and the development of arrays may have been synchronous rather than models of strain and fabric histories in broad sequential. Holdsworth et al. revisit the northern zones of ductile deformation. Coward's work sig- part of the Moine thrust belt, developing the clas- nificantly advanced the understanding of the sic notion that folds and other minor structures large-scale structural evolution of the continental have only local rather than regional significance crust through the application of shear zone in mylonite zones, and reassess the relationships models. With the development of new geophysi- between these structures and the major thrust cal imaging techniques, there is a renewed inter- surfaces. The internal deformation state of thrust est in the ductile deformation of deep continental sheets is a theme also addressed by Vitale et al. crust. Consequently, kinematic models based on Coward's conclusion that deformation processes simple shear for deformation zones in crystalline in the higher parts of the Moine thrust belt basement are being re-examined. Like Coward involved important components of quasi-coaxial before him, Wheeler uses the Lewisian of NW stretching is mirrored in this study, although the Scotland as an outcrop analogue for deep crustal example here is from the Southern Apennines of deformation processes but presents data from the Italy. Partitioning of different degrees of simple Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 5 shear and stretching are related to alternating to plate kinematics was one of Coward's central competency contrasts in the carbonates of the research themes. Nem~ok et aL look at the thrust sheet. regional evolution of the Carpathians, using vein Coward's work on the larger-scale signifi- arrays to estimate the orientation of palaeostress cance of structures in the Moine thrust belt fields. Aeosta et al. continue the tradition in their identified a series of faults and shear zones that study of the Colombian cordillera. For these appeared to accommodate regional extension researchers, the large-scale convergence associ- (e.g. Coward 1983). He saw the same phenom- ated with the subduction of the Nazca Plate is enon in younger systems with his recognition of differentially partitioned at different lithospheric large-scale extensional relationships within the levels and is manifested by a complex history of otherwise contractional orogenic belt of the NW strike-slip and thrust faulting. An analogous Himalayas (Coward et al. 1987). Presenting a theme is developed for the whole of the South range of kinematic data derived from linked field American continent by Cobbold et al. A series and petrological studies, Treloar et aL discuss of scaled physical models are presented as ductile extensional flow in the NW Himalayas. analogues for this deformation, showing how Rather than invoke the currently popular varying plate kinematics acting on the edge of the channel flow models (e.g. Beaumont et al. 2001), continent drove faulting in the orogenic interior these workers suggest that extension has been over the past 100 Ma. polyphase in nature, in part resulting from During the 1980s, Coward began to see the re-equilibration of the orogen following inferred importance of commercial seismic data in struc- slab break-off. The theme of extensional tecton- tural geology, beginning a series of investigations ics and the coeval exhumation of deeply buried of sedimentary basins that continued for the rest crust is continued in the following two papers. of his career. Commonly, complex structure in Al-Wardi & Butler present new field data from sedimentary basins is associated with the pres- the northern Oman Mountains, where extension ence (and mobility) of salt. Building on regional in the weakly buried Arabian continental crust at studies, such as appraisal of the South Atlantic shallow levels in the orogen can be traced down continental margins by Coward et al. (1999b), a palaeo-subduction zone into ductile shear in Davison presents a review of salt mobility and adjacent metamorphic terrains. Complex inter- basin evolution in offshore Brazil. He demon- actions between folding and extension on a vari- strates the importance of regional tilts, governed ety of scales can be explained by the differential both by tectonics and sediment loading, in influ- shear model of Coward & Potts (1983). Bond encing salt mobility and the consequent large- et al. address the kinematics of ductile extension scale gravity collapse structures. This style of salt within the Cyclades, drawing on field studies tectonics on a continental margin is different on the island of Syros. Harking back to Coward's from that of more symmetrical intracontinental (1986) discussion of continental extensional basins, and Davison's paper provides an inter- tectonics, these researchers provide a field study esting counterpoint to Stewart's review of salt that documents heterogeneous quasi-coaxial tectonics in the North Sea. This is a 'users' guide' stretching associated with the exhumation of to seismic interpretation using growth strata to blueschists and eclogites. The theme links back deduce the development of salt structures. The to discussions of the kinematics of distributed modern concepts of salt tectonics, especially the deformation in basement terrains, such as the role of differential loading and subtle tilting, are Lewisian. well illustrated here, as is the role of fundamental In the next group of papers, which deal with basement structures in governing the pattern of regional tectonic problems, Daly revisits the salt structures across the basin. Irumide belt of central Africa, drawing inspira- The role of basement structures in controlling tion from Coward's regional shear zone models the geological history of sedimentary basins was (e.g. Coward 1984). He shows the importance one of the great themes for much of Coward's of the basic framework of structural geometry work through the 1990s. In this volume several for the integration of diverse geological data. A papers take many of the classic inversion tectonic synthesis of field geology and geochronology is concepts (e.g. Coward 1994, 1996) and apply used to develop a history of arc accretion in the them to unusual settings. The oblique reactiva- assembly of continental crust. The study makes tion of pre-existing rift structures is explored an interesting counterpoint to case histories of through a series of physical models presented Tertiary orogenesis, such as the account by by Mattioni et al.. They show that under a con- Robertson et aL of the eastern Taurus Mountains stant far-field stress regime, complex polyphase of Turkey where the history of distinct crustal structural histories may be generated in blocks can be deduced from stratigraphic data. transpressional settings. Casini et al. apply ideas Relating local structures, such as fault systems, of inversion to an outcrop-scale linked system of Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

6 R.W.H. BUTLER ET AL. net-contractional faults, arguing that they repre- geology and basin evolution in mineral explora- sent a microcosm of structural evolution in the tion, an area where such an approach was not northern Apennines. Sepehr & Cosgrove also previously the norm. It is therefore appropriate discuss the role of structural inheritance, but that the volume concludes with case studies asso- apply the notion to parts of the Zagros fold belt. ciated with mineral exploration. The first two of The later papers in the volume are what might these deal with the Witwatersrand basin of South be called 'applied', and remind us that much of Africa, building directly upon Coward's work. Mike Coward's later work was for the oil and Beach & Smith present a series of cross-sections gas and mining industries. Cooper undertakes a that document the structural relationships and major review of hydrocarbon prospectivity in evolution of the basin, explicitly presenting their foreland fold and thrust belts, a study incorpo- conclusions in terms inherited from the world oil rating information from over 2900 fields world- industry, straddling processes and scale. JoUey wide. He shows that there is no single structural et al. also emphasize the importance of fault reac- factor that makes hydrocarbon systems work tivation in the Witwatersrand basin and discuss in thrust belts; rather, it is a combination of the impact on the architecture of individual fault attributes unique to each example that governs zones. The fact that the region has experienced prospectivity. He makes it clear that the lessons contraction, followed by extension, means that drawn from case studies must be applied with individual fault zones commonly record a com- care. A critical risk in many thrust belts is the posite history, arguably controlling the distribu- relative timing of structural development and tion of gold mineralization. Muntean et al. use hydrocarbon migration. This issue is directly models for inversion tectonics to understand the evolution of faults and folds, associated with the addressed by Sassi et al. in the Magdalena foreland fold and thrust belt of Colombia, Palaeozoic Antler Orogeny of central Nevada; the faults subsequently served to focus gold through integrated modelling of structural evolu- mineralization in Tertiary times. tion and petroleum systems. They show that Muntean directly acknowledges the role of his whereas one geological history predicts that the co-author, Mike Coward, for 'opening his eyes to target structure is prospective, in another the the forest as well as the trees'. It is a sentiment source rock is over-mature and the reservoir is that we know will be shared both by the contri- destroyed. In many structurally complex settings, butors to this Special Publication and by the small-scale structures, such as fracture networks, geological community at large. play a major role in reservoir performance. Too often, however, reservoir modelling studies are based on theoretical considerations alone. This collection of papers written by Mike Coward's There remains a paucity of field-based analogue former students, friends and colleagues arises from a studies, such as those presented here by Belayneh conference held in his memory at the Geological Society in May 2004 and an associated field trip to the High- et al., which constrain the fracture patterns. lands and Islands of NW Scotland. We thank all those Those workers use chalk outcrops of SE England who participated in these activities. to populate a reservoir model, to investigate structural controls on fluid flow, especially zones of concentrating fracturing (fracture corridors), References linking these to larger-scale structural evolution. The geometry and evolution of structures ARGAND, E. 1924. La tectonique de l'asie. Brussels, 13th International Geological Congress, 1, 170-372. influences fluid flow in settings other than those BALLY, A. W., GORDY, P. L. & STEWART, G. A. 1966. associated with hydrocarbon migration. Sibson Structure, seismic data and orogenic evolution of examines hydrothermal systems and related gold Southern Canadian Rocky Mountains. Bulletin of mineralization within the context of earthquake Canadian Petroleum Geology, 14, 337-381. models. He recognizes that most of the world's BANKS, C. J. & WARBURTON, J. 1986. Passive roof hydrothermal gold deposits are associated with duplex geometry in the frontal structures of the greenschist-facies metamorphism and therefore Kirthar and Sulaiman mountain belts, Pakistan. developed in the vicinity of the 'brittle~luctile Journal of Structural Geology, 8, 229-237. transition'. He speculates that it is the undula- BEACH, A. 1981. Thrust tectonics and crustal shorten- tions in the depth of the brittle-ductile transition ing in the external French Alps based on a seismic cross-section. Tectonophysics, 79, TI-T6. that exert control on the migration of over- BEAUMONT, C., JAMIESON, R. A., NGUYEN, M. H. & pressured metamorphically derived fluids in the LEE, B. 2001. Himalayan tectonics explained by crust, especially during the development of major extrusion of a low-viscosity crustal channel coupled brittle thrust faults. to focused surface denudation. Nature, 414,738-742. During his later career, Coward did much to BOSELLINI, A. 2004. The western passive margin of promote the application of large-scale structural Adria and its carbonate platforms. In: CRESCENTI, Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

INTRODUCTION 7

U., D'OFFIZI, S., MERLINI, S. & SACCHI, R. (eds) Thrust and Nappe Tectonics. Geological Society, The Geology of Italy. Societh Geologica Italiana, London, Special Publications, 9, 275-292. Special Volume, 79-92. COWARD, M. P. & POTTS, G. J. 1983. Complex strain BOYER, S. 8z ELLIOTT, D. 1982. Thrust systems. AAPG patterns at the frontal and lateral tips to shear zones Bulletin, 66, 1196-1230. and thrust zones. Journal of Structural Geology, 5, BREWER, J. & SMYTHE, D. K. 1984. MOIST and the 383-399. continuity of crustal reflector geometry along COWARD, M. P., NELL, P. R. & TALBOT, J. 1992. An the Caledonian-Appalachian orogen. Journal of the analysis of the strains associated with the Moine Geological Society, London, 141,105-120. Thrust zone, Assynt, Northwest Scotland. In: BUTLER, R. W. H. & COWARD, M. P. 1984. Geological MITRA, S. & FISHER, G. W. (eds) Structural Geology constraints, structural evolution and deep geology of Fold and Thrust Belts. 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