sign in sign up
<<
Home , Fold (geology), Geodynamics, Geology, Orogeny, Rock mechanics, Tectonics

Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Orogeny through : an overview

JEAN-PIERRE BURG & MARY FORD Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland

Abstract: As an introduction to a diverse set of papers on orogenic studies we a personal overview of the most notable developments in orogenic studies since the 1960s. The impact of new techniques such as deep reflection seismic profiling, , analogue and numerical modelling is discussed. The major geodynamic models and concepts which have stimulated orogenic studies in recent are also considered.

Orogenesis is the most complex of tectonic Changing approach to the study of orogens processes and interpreting ancient belts is one of the greatest challenges The major controversies that have arisen from face today. As originally defined by Gilbert the study of looking at orogens in the are (1890), an (from the classical Greek documented in several works (e.g. Cady 1950; 6ros meaning 'mountain' and gen~s meaning Condie 1982; Miyashiro et al. 1982). Based on a 'stemming from') is simply a of mountain mixture of spiritual contemplation and obser- building. To geologists the orogeny vation, many nineteenth century on the represents a penetrative of the origin of mountain belts could not conceive of 's associated with phases of meta- any large movements in the Earth to produce morphism and igneous activity along restricted, orogenic belts (fixist theories). The important commonly linear zones and within a limited time concept of lateral compression gained credence interval (Dennis 1967). However our increasing as late as the middle nineteenth century. It was understanding of the of only in the twentieth century that Earth scien- allows geologists today to view orogenesis on a tists first suggested (e.g. Argand 1924; Wegener larger scale as the interaction of a series of 1912; Wilson 1966) and then established with the geodynamic processes. This volume has arisen plate tectonic paradigm (e.g. McKenzie & from a seminar series given by invited speakers Parker 1967; Isacks et aI. 1968; Le Pichon 1968; in 1994 at the Geological Institute, ETH, Morgan 1968) that large horizontal movements Zurich. The aim of this lecture series was to were responsible for orogens. As provoke discussion on, and greater awareness elaborated elsewhere (e.g. Condie 1982) plate of, the larger issues of orogenesis. In particular, unified several long-lived theories such is there a change in style or mode of orogeny as those of and through geological time, or is variety of orogenic and rendered obsolete notions such as - features rather reflecting different wide orogenic cycles and a contracting Earth. and boundary conditions in . The wealth of Shortly after the acceptance of the plate tectonic data and ideas presented in this lecture series are , modern and ancient mountain belts were compiled here as a series of review-type papers. analysed in terms of global tectonics (e.g. This book can provide only scattered ex- Dewey & Bird 1970; Dickinson 1971). However, amples of orogens through time. Figure 1 shows partisans of primary vertical tectonics resisted the global distribution of of different the paradigm and maintained ages and those covered by this book are marked. that ancient orogens were better explained by Many of the Cenozoic orogens which have contraction of intracontinental mobile zones received considerable attention in recent years (evolved from ensialic zones i.e. (: Roure et al. 1990; Pfiffner et al. 1996; or geosynclines) between stable (cra- Schmid et al. 1996; : Treloar & Searle tons, e.g. Weber 1984). Zwart (1967) em- 1993; Pyrenees: Choukroune et al. 1990; Oman: phasized differences between the Alpine and the Robertson et al. 1990) are not covered in this Hercynian orogens, leading to the widely used volume. classification of orogens as either Alpinotype or

From Burg, J.-P. & Ford, M. (eds), 1997, Orogeny Through Time, Geological Society Special Publication No. 121, pp. 1-17. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

2 J.-P. BURG & M. FORD

6

~ and Cenozoic orogenic belts ~Palaeozoic orogenic belts Platforms ~ ~ ~. Crat ons ~ ___/Shields/

Fig. 1. of orogens distinguished by their age. Orogens coved in this volume are boxed. 1, Archaean orogens of Choukroune et al. a, Dharwar and b, Superior province; 2, Mount Isa , O'Dea et al; 3, the , Milnes et al. and Rey et al. ; 4, the Lachlan belt, Gray; 5, the Urals, Puchkov; 6, the Variscides, Rey et al.; 7, the Central , Lamb et al. Adapted from Miyashiro et al. (1979) and Condie (1982).

Hercynotype depending principally on the Burke 1973; Krrner 1981; Hoffman 1989). amount of , high- meta- Other researchers doubt whether Middle Pro- morphic rocks and . Application of the terozoic (1000 Ma) and older orogens could re- plate tectonic concept was however more fruitful sult from a plate tectonic regime (e.g. Hargraves in that geologists could show that major charac- 1976; Wynne-Edwards 1976; Reed et al. 1993). teristics of an orogen (namely deformation, It is often argued that geothermal gradients were and igneous activity) record much higher in the Archaean than at present be- stages of the plate tectonic of the orogen, cause heat-producing elements were much more i.e. successively , , collision abundant. Geological evidence for this comes and eventually post-collisional intra-continental from very high-temperature , komatiitic deformation. Many articles and books, dedicated in greenstoncs ~nd large plutonic bodies. to the link between plate tectonics and mountain The lithospheric plates would then have been belts (e.g. Mitchell & Reading 1969; Coney 1970; thin and their density too low to cause buoyancy- Dewey & Bird 1970; Dietz 1972; Gilluly 1973), driven subduction. However, vigorous have convincingly revealed that modern orogenic could have resulted in folding and belts occur principally at convergent plate faulting of the thin (reminiscent of boundaries and result from collision between the contraction theory). This heat- and gravity- continental, arc-derived or oceanic crustal driven activity would indeed have triggered blocks. intraplate orogenesis. However, the argument Today, it is generally agreed that plate may simply imply a secular variation in style of tectonics were acting throughout Phanerozoic intraplate deformation which does not exclude time, although deduction of relative directions plate boundary deformation, because if plates and rates of continental drift before 200Ma did exist in the Archaean, they would have been remains a problem. Some authors extrapolate the smaller and their motion twice as rapid as Phan- theory to the whole Precambrian (e.g. Dewey & erozoic rates (Sleep & Windley 1982). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 3

Recent techniques DePaolo 1981). The additional information gained from dating lithospheric plates facilitates Since the 1960s the impact of and the definition of mineralogical and physical geophysical data on the study of orogens has differences that may develop with time. been enormous. Rapid advances in technology and increasingly powerful computers have (1) Seismicity and deep reflection seismic profiling: generated completely new data sets (e.g. geo- the layered lithosphere. The layered configur- and deep seismic profiling) which ation of stable lithospheric plates verified by must be reconciled with more traditional field seismic studies stands as a fundamental concept observations and (2) allowed numerical model- that rules the mode in which an orogen may ling of complex Earth systems and processes evolve. Seismic studies have used S- and P-wave whose results can be compared with and con- velocities to recognize a layered Earth with a strained by factual data. Field geologists can thermally conductive lithosphere overlying the increasingly interpret their data in terms of large seismic low velocity zone of the convective scale lithospheric or crustal processes. We mantle. Thus a plate is colder and therefore summarize below what we feel are the more more rigid than the underlying . important of these modern techniques and Lithospheres are themselves seismically layered models and the impact they have had so far on and consist of an upper rigid layer and a lower the study of orogens. viscous thermal boundary layer (Parsons & McKenzie 1978). The crust is the upper part of Geochronology. Isotopic techniques permit the the 100-150km thick mechanical lithosphere. dating of crystalline rocks and therefore have Seismic velocity-depth models simplify as one become a prerequisite to understanding the level of bulk tonalitic composition the 20-35 km crystalline axes of all orogens. Our knowledge, thick , although geologists particularly in the Precambrian regions, owes know it to vary- greatly in lithological content. much to these methods. For example, many Lithospheric layering varies in young and old , initially recognized as Archaean from regions, which is particularly important for the high- metamorphic and abundant plutonic mechanical behaviour of the plate, therefore the rocks (a association long used to identify resistance to long term . It also varies in the Archaean) were revealed to be much time since the crust may reach thicknesses of younger by techniques. A 60-70 km under high . Less is known spectacular example is the so called 'Tibetan on the thickening processes and lower limits of ' believed to be Precambrian until the mantle lithosphere that can be subducted or its granites were dated as (e.g. Gansser thickened to 200km such as under the Alps 1964, 1983; Deniel et al. 1987) intruded into (Panza & Mueller 1979). rocks that have undergone an extensive Ceno- The advent of deep seismic reflection and zoic metamorphism (e.g. Le Fort 1975; Brunel refraction profiles has better delineated two- 1986; Brunel & Ki6nast 1986). In this case, the dimensional structural details of the crust. Since advent of absolute dating has dramatically the 1970s orogens of all ages have been imaged changed the interpretation of the orogenic by deep seismic reflection profiling (e.g. BIRPS system and further examples can be expected in the , ECORS in France, DE- from dating of so-called microcontinents KORP in Germany, COCORP in the USA, or 'Zwischengebirge' (Kober 1928) within Lithoprobe in and NFP20 in Switzer- Cenozoic belts. However different radiometric land), which has again highlighted the layered methods ages with different meanings (e.g. of the lithosphere. The bilateral sym- Fowler 1990, chapter 6) so that the true metry of orogenic structures, often emphasized significance of some isotopic data is still ques- in the past, is now considered unreal. Particu- tioned. Do isotopic ages constrain the sequential larly illustrative in this respect is the manner in development of growing or waning stages of an which the Pyrenees have been portrayed before orogen? and after the ECORS deep seismic profile has also resulted in the (Choukroune et al. 1990). In the Central Alps division of large into provinces sur- the symmetrical 'Verschluckung' models (Laub- rounded by younger orogenic belts with an age scher 1974) have been replaced by a strongly progression away from the oldest central craton asymmetrical model as imaged on the NFP20 (e.g. Hoffman 1988). This distribution is inter- profiles (Fig. 2) showing subduction of the lower preted as recording the coalescence of arc European crust southward into the mantle systems, hence implying subduction and (Valasek 1992; Schmid et al. 1996). eventual collision between the provinces (e.g. Low-angle reflectors are conspicuous features Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 J.-P. BURG & M. FORD

N ,, / I nsubricline S

Helvet,c Austroalpine ] LLljjjj L[I]j ILl ~1'

crust 0 25 50 75 100 125 150 175 km I I I I I I .:. I I 0 ~ ..... : -= - ~.- ~ --~ .. ]~.....~ . . . - . .~ :. - . .- - - ..,,.,~ - :-- . ,.- - - .- ~, .. ,,, . -. - .. -- ,,v,- ~.- .,t ~ .

10 "-'.. -' --,,: :--, - .7- :.-- ---:-.~...,, .. :",-.-.,,- " . - - r,,~zz_3a,,, .'-"~-2 - -..."a,~.'~,." . ,. -.., .. ;;z-'.'. ~-.." .-,-a--,. :.

.--.r at';. -1,,~ " • .... --. "..,,,~.~,,~-~..- ,,-~ ~. ~',,.:,-- ,',,;,.- : ,'¢--'7--.,~" t~,,~- -..z.~t,'... -- ~ ".,.~" •

50- '., .,, -: . --.-f.fz " .-. . . " - ~=-.-.-~_E.~..Q-..-- ..... ~-. ::.~""L" :- - -'-_':. " - .... : ".'7-. - ". " - ~. --. " • ". - -- . -7" - --~--".~.~ .'--. :~-..:.,: 2->- . _:- :: - " .'. ., -- :, .. - . .- ...... ".---" :-.-,:~...,~.,.~ ] . . . . ,. 60- -~ --'- " . - " -.." .-"-. "" 5:';~- -:].--~'~.,..=-":,;:.: :'. ' • . . -- . . -.. - - -'.~. ,:_ ~:-,r.. ..- , • ~ ...\ . . ,. : ,~..,..._. o • . .. , ;. ; "'-.[~ ..a~. . 70- depth (km) Eastern Swiss Alpine transect - " ~,. " ~ . ""~"M'

I I I I I I i

Fig. 2. Depth converted line drawing of the NFP20 deep seismic profile through the Central Alps. Adapted from Valasek (1992). The corresponding surface geology is shown above the profile. M-M' marks the trace of the European Moho which clearly plunges down below the . identified in most orogens. They are interpreted 1995, this volume; Ranalli & Murphy 1987). as thrust zones possibly connected with crustal- Temperature is the main controlling parameter, scale drcollement zones (Cook & Varsek 1994) which is itself controlled by the thermo-tectonic that show the bulk asymmetry of orogens. age and composition of the crust, in particular Upper crustal faults cannot be traced downward the amount of radiogenic elements. Therefore, into the ductile lower crust (e.g. Meissner 1989). strength profiles differ for lithospheres of differ- Sharp irregularities of the Moho support the ent ages (old lithospheres are 'cooler' than idea that the is often brittle (Hirn young lithospheres) and origin (i.e. oceanic or 1988; Goleby et al. 1990). continental). The character of an orogen as depicted by large scale structures will obviously Rheology: strength of the lithosphere. The be dictated largely by the rheology of the mechanical properties of lithospheres have been lithospheres involved. It is worth noting that explored from inferred mineralogical stratifi- undisturbed Moho is known in obducted lithe- cation, temperature gradient and pressure con- spheres (Nicolas 1990), which suggests that ditions in the plate (Kusznir & Park 1984; rheological layering is not necessarily coincident Ranalli & Murphy 1987). The estimated rheolo- with this seismically defined boundary. gies are primarily based on extrapolations of It is generally accepted that the lithosphere mechanical properties of and rocks has a mechanical strength which can evolve over obtained in high-temperature high-pressure geological time. This strength is defined using an laboratory (e.g. Kirby 1985; Carter elastic layer whose thickness depends on ther- & Tsenn 1987). They suggest that the cold upper mal gradient and crustal thickness (hence age, continental crust (-feldspar dominated) is e.g. Kusznir & Karner 1985). The effective brittle and deforms dominantly by faulting. elastic thickness of continental lithospheres has Below 10-20km (300-400°C), the continental no geological reality (Burov & Diament 1995). crust flows and deforms by solid-state creep rather than by . The uppermost part of Lithospheric modelling of orogenesis. Two the olivine-dominated mantle is also brittle and modelling approaches have been used to investi- may undergo faulting. The lower levels of the gate mountain building processes: numerical lithospheres are ductile. Corresponding strength and analogue modelling. Both point to the profiles have several maxima and minima (Car- fundamental control exerted on the deformation ter & Tsenn 1987), which image the lithosphere style by the strength of the crust and its coupling as a rheological multilayer medium (Ranalli with the rigid or ductile mantle at its base. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 5

Block Uplift Pro-wedge Retro-wedge

crustal subduction (1/3 crust subducted)

Fig. 3. Lagrangian grid showing deformation of the crust generated by a two dimensional plane strain finite element model of a small compressional orogen. Redrawn from Beaumont & Quinlan (1994, fig. 6) by kind permission of Blackwell Science, Oxford. This model represents a cold, single layer crust where one third of the crust is subducted. The crust is modelled with the rheology of wet feldspar. Decoupling of the lower one third of the left-hand crust occurs at the singularity (black spot). The model is shown after 2 Ma and 40 km of convergence. These geometries are comparable to those seen on deep seismic lines through the Pyrenees (ECORS) and the Gulf of Bothnia, Svenofennide transects (Beaumont & Quinlan 1994) and possibly with the Central Alps (compare with Fig. 2).

Finite element models use continuum mech- gravitational forces. As a consequence, with the anics to calculate crustal deformation patterns present- accepted lithospheric , the ( & McKenzie 1983; England & House- crust may hardly thicken homogeneously more man 1986; Vilotte et al. 1986; Cloetingh et al. than 60-80 km (Dewey 1988; Molnar etal. 1993). 1989). The crust is considered to have plastic and Therefore, metamorphic equivalent to viscous rheologies. The continuum deformation 100km in collisional belts (e.g. Chopin 1984; calculated incorporates rheological layering and Harley & Carswell 1995; Schreyer 1995) are coupling between laterally uniform layers. The likely to be related to subduction of continental method has been particularly successful in material below all possible Moho levels. understanding the dynamics of lithospheric Lithospheric analogue models of continental processes in two-dimensions. Results commonly shortening generally involve a strong, brittle illustrate that the style of crustal deformation upper crust, a weak ductile lower crust and a depends strongly on geometrical boundary con- strong mantle floating on a fourth, low-viscosity ditions such as symmetry or asymmetry of layer simulating the asthenosphere (Davy & mantle shortening to produce symmetrical or Cobbold 1991; Cobbold & Jackson 1992; asymmetrical orogens, on properties of the crust Shemenda & Grocholsky 1992). Analogue ma- and on the amount of crust that can be subducted terials are scaled and layered to reproduce the with the mantle (Fig. 3). strength variability of lithospheres. They are Every description of collisional orogens points particularly useful in investigating in three to folding and thrusting as the principal mechan- dimensions the development of large-scale litho- isms of crustal thickening coeval with horizontal spheric structures (Fig. 4). Relevant conclusions shortening. These structures record regional are (1) the thickening style depends mainly on deformation but are too small to be taken into upper mantle behaviour. Lithospheres with a consideration in terms of thin-sheet approxi- ductile upper mantle (thus reduced to two-layer mation, i.e. these structures can be smoothed systems) thicken more symmetrically and homo- into a bulk homogeneous strain of the litho- geneously than those with a brittle upper mantle sphere (England & Houseman 1986; Houseman (four-layer systems), (2) the lithospheric thick- & England 1986). It frustrates geologists to ness controls the width and distribution of know that their observations and measurements deformation zones, (3) non-isostatically com- become local, mechanical anecdotes and noise pensated buckling may occur in the initial stages when it comes to understanding collisional of shortening and controls the strain localization orogeny in terms of lithospheric deformation. with further shortening and coeval thickening, First-order approximation is however the best which is strongly controlled by the coupling of means of appreciating how much a continental brittle layers through the ductile intermediate crust may thicken. Calculations show that layers (Davy & Cobbold 1991). gravitational forces increase 'non-linearly' as crustal thickness and elevation increase Recent geodynamic concepts (England & McKenzie 1983). Both elevation and crustal thickness are buffered by the ratio of Some new geodynamic concepts have been the force needed to drive convergence and developed in recent years that are not fully Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

6 J.-P. BURG & M. FORD

(a)

(b) Fig. 4. An analogue model of a mountain belt after c. 30% shortening using a four-layer lithosphere (from Davy & Cobbold 1988). (a) side view of the shortened belt with a high topography and a root zone floating on a weak 'mantle'. (b) perspective view of the top surface of the model showing topography. Oblique stripes are passive markers to highlight thrust discontinuities and relative displacements between crustal segments.

integrated or addressed in the articles that make more problematical. Several large-scale geody- up this volume. These concepts deal with very namic models for orogenic processes have been large scale of orogens, usually far proposed in recent years that, while addressing larger than the structures recognised and this problem, also have a wider impact on mapped by geologists. We summarise some of orogenic studies. These are the critical wedge these concepts because we believe that they have model, the doubly vergent critical wedge model, an important impact on orogenic studies and the corner flow model and the subduction should inspire future interpretations. channel model. Chapple (1978) equates the deformation and Critical wedge theory and the of high displacement of a thrust sheet to a plastic layer pressure metamorphic rocks. The presence of which will deform to become a wedge-shaped very high-pressure rock units in orogens is one of continuum ahead of a rigid buttress (or back- the principal geodynamic problems in orogenic stop) and above a non-deforming, subducting studies (Platt 1993). Ultrahigh-pressure crustal slab. Davis et al. (1983), Dahlen et al. (1984) and rocks have been found in many orogens indicat- Dahlen (1984) extend this to materials ing that crustal segments have been subducted to with Coulomb-type (brittle) behaviour, ap- depths of 70-100km and have then been plying their models to accretionary prisms and exhumed rapidly enough to preserve the high- external fold and thrust belts of orogens. The pressure associations (Coleman & wedge (Fig. 5) must constantly adjust in order to Wang 1995). It is now accepted that crustal rocks achieve dynamic equilibrium, i.e. to reach and can be subducted on the downgoing slab to great maintain a stable shape (critical wedge) for depth; however the exhumation process is which the gravitational forces balance the Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 7

xX\\ X X \ EROS,ON X EXTENSION

SUBDUCTINGPLATE

Fig. 5. Sketch representing the critical wedge model and the main processes that contribute to the dynamic system. This model can be applied to a single thrust sheet, an accretionary , a or to a whole orogen, a is the dip of the upper surface which will be affected by all the processes noted (see text for discussion). 13 is the dip of the subducting plate. Inset shows the corner flow model for a subduction complex. Arrows indicate the path followed by exhumed high pressure rocks.

traction exerted on its base by the subducting grade rocks within a single geodynamic system. slab across a zone of low . The basal angle Thus the critical wedge model has come to play is maintained by the dip [3 of the subducting slab. an important role in tectonic studies of orogens The stable geometry generates a surface slope oL (Platt 1993). in the direction of relative displacement. The A mechanical model for doubly-vergent '' defines the angle at the apex of the orogens (such as the Alps, the Pyrenees) has wedge (~ + [3), which relates the wedge shape to been developed (Willett et al. 1993) which the compressive and gravitational forces during removes the need for the poorly defined back- subduction (Davis et al. 1983). Material is added stop or buttress (Fig. 3). The model is based on to the wedge by frontal offscraping (reducing a) numerical modelling backed up by sandbox and by underplating, the addition of material to modelling. Shortening and deformation of the the underside of the wedge (increasing o0. crust occurs above two convergent, nearly rigid, Internal horizontal shortening is achieved by mantle plates, one of which subducts into the thrusting and folding, which produces thicken- asthenosphere. Many of the large scale geologi- ing (a increases). Then the wedge may extend cal processes active in orogens can be repro- (decreasing ~), through listric normal faulting, duced and deep seismic profiles through many to regain stability. reduces e~. orogens show the asymmetry predicted by this Platt (1986) applied the critical wedge concept work (Beaumont & Quinlan 1994). to whole orogens in order to investigate the Movement of material within a subduction geodynamic processes by which high pressure complex has been modelled using viscous rheolo- metamorphic rocks can be exhumed during gies for material caught between the subducting orogenic convergence. This application allows plate and the hanging wall buttress (Fig. 5, inset; consideration of seemingly diverse processes Cowen & Silling 1978; England & Holland 1979; such as the intricate association of syn- Emerman & Turcotte 1983). This 'corner flow' metamorphic convergent deformation, decom- model has been proposed as a mechanism for the pressional extension and exhumation of high- exhumation of high-pressure rocks although Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

8 J.-P. BURG & M. FORD N S 1 = Buckling

2 = Thrusting and amplification

• . ~ ...... ~..:~..:~ ~..~ ~.... ,,,,,

3 = Trough contraction INDIA ~ TIBET TARIM TYAN SHAAN SIBERIA

Fig. 6. Successive stages of lithospheric buckling inferred from analogue modeling used to interpret the Cenozoic intracontinental shortening/thickening of after collision with India (adapted from Burg et al. 1994a). Shaded layers indicate the brittle upper crust and upper mantle. Decoupling at mid crustal level would take place along a partially molten horizon and gives rise to mechanical instabilities (buckles) with unrelated characteristic wavelengths in the lower and upper brittle layers. Conjugate thrusts, 'crocodile' wedges and Moho topography are formed. significant incompatibilities have been found the mechanical lithosphere (Martinod & Davy between the model and nature (Platt 1993). 1992, 1994). The subduction channel or flow-channel Lithospheric buckling has seldom been for- model (Cloos 1982) assumes that material mulated in the geological literature (Lambeck contained in a low viscosity mud matrix is 1983; Hoffman et al. 1988; Stephenson et al. squeezed in the wedge-like zone between a 1990; Nikishin et al. 1993). However it could be subducting plate and the overlying rigid ac- very important during the early stages of cretionary complex. Fragments are plucked intraplate shortening. In particular, buckling from the underside of the wedge and carried to may geometrically control the location of the great depth in the lower part of the flow-channel crustal scale thrusts that form in the inflexion by the descending plate. In low-angle (corner) zones of the initial buckles to accommodate wedges, a forced and upward return flow carries strong shortening (Fig. 6), and may trigger this material slowly back to the surface, where it conjugate thrust faults and full ramp com- is deposited at the toe of the accretionary prism. pressional basins in an overall asymmetric This model can be applied to areas where high system (Cobbold et aI. 1993; Burg et al. 1994a). pressure metamorphic fragments are preserved Numerical modeling indicates that lithospheric- within a tectonic mrlange such as the Franciscan scale folds are expected when the lithosphere complex in but does not explain larger possesses a brittle-ductile stratification scale high pressure metamorphic com- (Stephenson & Cloetingh 1991). plexes within orogenic belts (Platt 1993). Decoupling within the lithosphere. Decoupling Lithospheric buckling followed by failure and due to elevated pore pressure remains the best thrusting. Calculations assuming an elastic plate explanation for weakly deformed thrust sheets suggest that lithospheric buckling is impossible and accretionary prisms that have been dis- (Turcotte & Schubert 1982). However, if the placed over long distances relative to their foot lithosphere behaves as a viscous, ductile layer wall. The process is obviously important during floating on a weak asthenosphere, lithospheric subduction in separating from their buckling seems possible (Biot 1961; Ramberg basement (von Huene & Lee 1983). On a larger 1970). In fact, growth of periodic lithospheric scale, several analogue experiments show that folds seems related to the plastic rheology of the decoupling between the upper and lower layers 'brittle' layers of the lithosphere, with strong of continental lithosphere may be an important coupling between these brittle lithospheric lay- and underestimated feature of crustal thickening ers resulting in buckling of the whole lithosphere (Shemenda & Grocholsky 1992). Decoupling (Stephenson & Cloetingh 1991). In com- within the crust would indeed facilitate different pression, the wavelength of the lithospheric structural styles in the upper, brittle layers of an folds is approximately four the thickness of orogen and in the lower, ductile root zone, even Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 9

mountain elevation- - - al stress ~m ~ountain ...... Ii, ,;-I- ~- +- + - -I -!- - vertical normal stress Plow', !,J p ",,i land

- i- ' ! L ' I ~beneath

depth of crustal root .... mantle depth,

Fig. 7. Qualitative plot of vertical normal stress with depth (adapted from Bird 1991, England 1992, Tapponnier & Molnar 1976). The averaged vertical stress, equated with lithostatic pressure, is larger beneath the mountains than beneath the neighbouring lowland at any given depth in the crust. The topographic load causes collapse and expansion of elevated mountains towards low altitude forelands. permitting antagonistic structures such as the nier & Molnar 1976; Bird 1991). However, in 'crocodiles' at mid crustal level (Meissner 1989; uniform Newtonian sheets, a marked decrease Burg et al. 1994a). Subduction of the decoupled in convergence rate seems necessary before lower lithospheric layers into the mantle as horizontal boundary forces are overcome by the imaged on deep seismic lines (e.g. Fig. 2) help to buoyancy forces (England & Houseman 1989). explain the balance of crustal material within There are regions where evidence clearly shows collisional orogens. D6collement can occur at that convergence continued during the ex- different levels within the lithosphere leading to tensional phase and therefore decrease in rate of variation in large-scale characteristics of orogens convergence can be eliminated as a sole cause (Willett et al. 1993; Beaumont & Quinlan 1994). for orogenic collapse. In these areas an increase Direct observation of oceanic Mohos suggests in the potential of the mountain belt can that this interface is not a decoupling surface be achieved by the removal of the thickened along which obduction can take place. lithospheric root. There are several numerical models for the process by which the mountain Late-orogenic extension - detachment of litho- root can be removed (Houseman et al. 1981; spheric roots. Late orogenic extension or col- England & Houseman 1989; Molnar et al. 1993; lapse has been detected in Phanerozoic Platt & England 1994); the convective removal mountain belts (e.g. Wernicke 1981; Coney of the lithospheric root and of the 1987; Dewey 1988; Andersen et al. 1991; Burg et mantle lithosphere are the two main mechan- al. 1994b) and has been suggested as an isms proposed (Fig. 8). The basic concept is that important process in the formation and ex- the mantle lithosphere is denser than the humation of Precambrian granulite- ter- underlying asthenosphere. Thus a thickened (Sandiford 1989). Several theoretical and lithosphere can become gravitationally unstable mechanical considerations have shown that causing the root to detach and sink into the within a collisional plate setting, thickening of asthenosphere, allowing the hot asthenospheric the continental lithosphere generates local in- material to rise (Fig. 8). Isostatic readjustment ternal forces and changes in horizontal forces leads to extension and the increased geothermal that can produce extension (Molnar & Tappon- gradient can lead to and the nier 1975; England & Houseman 1989; England production of K20-rich (Platt & & Molnar 1993). In other words, shortened and England 1994). Lithospheric delamination consequently thickened lithospheres may below the Alboran Sea and -Betic mountains undergo post-orogenic collapse to achieve equi- is inferred from teleseismic P wave residuals librium thickness (Dewey 1988). Body forces (Seber et al. 1996). generated in high elevation regions (Fig. 7) can be important enough to develop extension in a Indenter tectonics. The Cenozoic history of Asia convergent regime (Artyushkov 1973; Tappon- shows that plates are not quite rigid and Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

10 J.-P. BURG & M. FORD

(a) AFTER THICKENING (b) CONVECTIVE REMOVALOF LITHOSPHERIC ROOT

.++++++++++~,.,,-,,,,.,-,-.+++++++++~-T T T~+ + ++ + ++ ++ + ++ +++--~ TT ~ .+-;-.~++++++++++++++.~F~ + + + + + + + + + + + ++ + + +++++++--~ .... ~-,-~.j.~++++.~t.,~uo~.++++~ ,,,--.-~---~++++++++++++++++,,..,"~"'7'~,,,

5,%•5,%•.,%~.•%5,5,..%5,~...%5,%.~5,•5,~. 5,5,.~5,•5,~,-,.~.%'~5,%5,'~%'..%••".~.%%',.5, '-'-'~MANTLE LITHOSPHERE::p "~

CONVECTING • ASTHENOSPHERE-'~

(c) DELAMINATION OF (d) STRETCHING ABOVE THINNED MANTLE LITHOSPHERE MANTLE LITHOSPHERE

++++++++++ ~++++++++++++~ ÷ "t" "!" ~ ~++++++~ ~+++++++.+,-**-~ -I- + + + ~ + + + .~-~ 5Jr_ + +--# 5 ±~.._-l: x ~..~ ... 5, • 5, 5, 5, -,... -~ 5, • % 5, % --. • • ~ % 5, ~ ~ • ~t•/~/"p,,+ + + + + + + + + + + -I~t t~•~•s ~ ,lllltlllllllitllllllliliiJ Illll." II¢'!/I ltlllll, cx:x/•x/5,/x1• / /

partial melting @ of asthenosphere

Fig. 8. Models proposed to generate orogenic collapse by removal of the lithospheric root. (a) Symmetrically thickened lithosphere whose root can be removed by either (b) convection of the underlying asthenosphere or (c) delamination. After the root has detached, the asthenosphere rises and re-equilibration leads to (d) stretching of the thinned lithosphere and possibly partial melting of the asthenosphere and K20-rich volcanism. that shortening is not located only at their of continental blocks takes place towards free boundaries. Widespread deformation, and par- plate boundaries (Tapponnier et al. 1982). ticularly wrenching within the con- However neither the indenter nor the extrusion tinues after plates have collided (Molnar & models address the orogenic problem proper, Tapponnier 1975; Tapponnier & Molnar 1976) namely lithospheric thickening and subsequent and it is geologically difficult to distinguish thinning to equilibrium thickness. between orogenic deformation due to active tectonics at plate boundaries and persistent Crustal thickening as a cause for intracontinental deformation. The model of a topography. Geological and theoretical con- rigid block indenting into a plastic body (Molnar siderations as well as analogue modelling sug- & Tapponnier 1975) is quite successful in gest that collision proper, that is the time of describing and predicting the far stress and strain contact between the continental plates and the fields resulting from collision. The deformation early deformation that follows it, does not field is expressed by slip lines whose pattern is produce high topography. For example, adapted to the geometry of the deforming marine sediments as old as or younger than system (Tapponnier & Molnar 1976), a versa- collision in Tibet show that the Indus-Tsangpo tility that may find application to orogenic collision zone was below (e.g. Burg & systems as complex as that of the peri-Mediter- Chen 1984) at collision time. This is certainly ranean (Tapponnier 1977). Long lasting inden- true today in the Caribbean realm (Dercourt et tation is accommodated by large wrench faults al. 1993) and for the present day collision along which lateral extrusion (escape tectonics) between Timor and (Karig et al. 1987). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW ductile, weak lower crust collisional zones until intracontinental thrusting Upper crust~ \ Mantle layer with appears (Fig. 9, Chemenda et al. 1995). Buoy- sediments of the ~ ~ the same yield limit ancy forces may become dominant after enough . ,continental margin ~ \ as the upper crust continental crust has been subducted causing I,a) - "",,~~"~_~ \ sea exhumation of high pressure metamorphic rocks overriding plate ~'~"" ~k--~l"-~evel (Fig. 9). Thus high mountains seem to result from intracontinental deformation that follows collision rather than collision itself. Notably, the Himalayas overlie an intracontinental thrust, the Main Central Thrust, rather than the India - liquid asthenosphere I Asia as marked by the ophiolites (Gan- (b) .'-' sser 1964) and Tibet is nearly 40Ma younger than collision (Molnar et al. 1993).

Orogenesis: a geodynamic overview The question of whether orogenic processes have remained the same through geological time is directly dependant on the answer to the question: have the rheological and thermal (c) properties of the continental lithosphere changed through geological time?

Lithospheres: their rheological variations In this volume, Giorgio Ranani discusses the thermal and rheological properties of the lay- ered lithosphere and their importance to oro- genic processes, including gravitational collapse of softened lithospheres and detachment of lithospheric roots. Thermal considerations sug- Fig. 9. Line drawings of an analogue to gest that the lithosphere has significantly simulate subduction of the continental lithosphere. changed through time, yet not enough to discard The model is driven by a piston on the right hand side plate tectonics for the Archaean. Instead, plate and by the drag force of the subducting lithospheric tectonics probably operated at different rates. mantle. Al represents the amount of convergence. The buoyancy force grows as the amount of Even if plate tectonics were responsible for continental crust subducted increases. Finally the orogenesis, body forces exerted on hot litho- upper crust fails and is uplifted. Uplift continues until spheres were also important parameters that the buoyancy forces are balanced by friction against controlled the development of Archaean oro- the adjacent plates and by the weight of the part of genic structures. In other terms, there are some the sheet raised above sea-level. Erosion generates rheological grounds to infer that ancient orogens further uplift. A major normal is formed were not fundamentally different from modern between the uplifting block and the overriding plate. mountain belts, varying more in their rates and A comparison can be made with the Himalayas where structures than in the tectonic processes from the frontal thrust could be equated to the Main Central Thrust and the normal fault with the North which they derive. Himalayan Normal Fault (adapted from Chemenda et A new dimension has been given to terrestrial al. 1995). geodynamics by the burgeoning research on planetary tectonics. An example of the import- ance of such work is given in the paper of Pierre Theoretical consideration seems to account for Thomas and colleagues who approach the this feature: cold roots of thickened lithospheres mechanics of lithospheres through the geometri- are denser than the warm asthenosphere at the cal characterisitcs of impact craters. Interest- same depth. The weight of this root causes a ingly, they show, among other points, that (1) negative buoyancy that counters the isostatic there is a secular variation of the mechanical and uplift of the crust, hence diminishes the average thermal state of the outer layer (? lithosphere) of elevation (Molnar et al. 1987). At this of and icy bodies, an evolution which collision, slab pull may rule the system. Little therefore should be relevant to the Earth and (2) topography is also seen in analogue modelling of parameters other than lithospheric thickness Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

12 J.-P. BURG & M. FORD should control plate tectonics. These con- mum of 50% shortening by a complex sequence clusions therefore support the idea that Arch- of folding and thrusting was associated with aean orogeneses probably deformed LP-HT metamorphism and prolific lithospheres that had properties different from intrusion. High-grade rocks are concentrated in those measured today, which should be of some a crustal block bounded by zones. Thrusts consequence for orogen distribution, duration show no cratonic and there is no and size through geological time. metamorphic hinterland. The geodynamic processes which contributed Orogenic case to the Norwegian Caledonides, an Early Palaeo- zoic collisional orogen, are discussed by Geoff With the rheological approach in hand, the Milnes and coworkers using a transect through discussion now requires the description and the southern part of the orogen. Extreme crustal comparison of orogenic systems using case shortening was accommodated along low angle studies. Conflicting interpretations for geody- detachments in the upper crust while an over- namic processes in the Archaean arise partly deepened crustal root generated eclogites at from the techniques used to study these orogens depth. These authors argue that significant late as illustrated by the paper by Pierre Choukroune orogenic extension which in part exhumed these and colleagues, jointly written by two indepen- high and very high pressure rocks was not due to dant research groups. Pierre Choukroune and orogenic collapse but instead due to changes in his team have used basic structural field tech- plate motions. Patrice Rey and associ- niques to study the Archaean of SW India (the ates concur with this conclusion by comparing Dharwar craton) while John Ludden and his the characteristics of late orogenic extension in have combined surface data with deep the Scandinavian Caledonides and the Euro- seismic reflection data from the Lithoprobe pean Variscides. They argue that while the project in the Superior Province of Canada. This Variscan extension was due to horizontal buoy- paper perhaps demonstrates that problems in ancy forces acting on a thermally softened and reconciling differing interpretations may in part thickened crust (orogenic collapse), the E-W be due to the differing approaches and data sets late orogenic extension in the Scandinavian used. Caledonides was generated by a far distant strain The Proterozoic Mount Isa terrain of NE field due to N-S Variscan collision. Australia (Mark O'Dea and collaborators) ex- The Late Palaeozoic Urals, an orogen which perienced repeated phases of extension and has long been poorly known in the English compression. This is an intracontinental literature is described by Victor Puchkov. This orogeny generated without subduction. None of orogen records a Wilson-type cycle terminating its rifting events proceeded to the breakup stage. in the collision of the east European passive Instead, extensive rifting accompanied by mag- margin with the active Kazakhstan margin and matism was followed by shortening and later involving intervening arcs and crustal wrench faulting associated with LP-HT meta- blocks. The asymmetrical orogen encompasses somatism of the Isan Orogeny. There is no the typical features of a collisional orogen: a lateral migration of deformation and the most (), a fold and thrust belt, intense deformation, metamorphism and meta- an ophiolitic suture and a complex metamorphic somatism are concentrated in shear zones. The hinterland, fragments of volcanic arcs, a granitic rift system appears to have provided a weak axis. lithospheric zone in which later compression and Lamb et al. present a study of the Cenozoic wrenching could concentrate. The source of central Andes, a subduction orogen active since stresses is not clear. Either they were trans- the . The crustal thickness in places mitted through continental lithosphere from exceeds 70 km but the lithosphere is not thick- plate boundaries or they were generated by the ened. On the contrary, the authors show, using underlying convective mantle. isotopes, active mantle melting and The mid-Palaeozoic Lachlan fold belt in SE thinned mantle lithosphere above the subduc- Australia (David Gray) developed above an ting plate and directly below the main body of accretionary plate boundary. To the east lay the the Andes. Thus these mountains (the Altiplano long-lived active eastern plate margin of Gond- lies at c. 4000 m) exist in dynamic equilibrium wana. The Andean type orogen developed by above an active subduction zone. The crust has of sediments and magmatic complexes thickened by extensive , vol- to the plate overriding a subduction zone. No canism and minor intrusion and shortening by lower crustal rocks seem to have been exhumed thrusting and folding in the upper levels, with during this protracted orogeny, where a mini- synchronous magmatic underplating and ductile Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 13 deformation at depth. The area has gradually are too controversial to estimate the amount that risen due to these processes since the Cre- has been truly subducted in an orogen. This taceous. This case study documents extensive basic problem has received little attention from sedimentation within a compressional and up- geophysicists and the presently accepted values lifting orogen, not only in foreland basins but range from a few tens to a few hundred also in substanial intramontane basins such as kilometres (Molnar & Gray 1979). The amount the Altiplano. of subducted continental lithosphere remains an important research direction for students of Conclusions orogenesis because gravity, expressed through the negative buoyancy of subducted material, From a geodynamic point of view, orogenesis causes topographically high mountains and their encompasses all processes that result in crustal eventual collapse. thickening, causing uplift and hence high top- There seems to be no rule concerning the time ography. Whether the term implies thickening spanned by an orogeny. Modern examples teach of the whole lithosphere is an obvious and us that plate motions that produce mountain crucial point of contention. Although plate ranges may last for several hundred million tectonics is the main agent of orogenesis, years. This is shown by the, at least, 200 Ma old continental crust can thicken by horizontal subduction zones along the western margins of shortening or by vertical processes such as North and . Alternatively, magmatic underplating and granite intrusion several cumulative short-lived phases of motion (e.g. Dewey & Bird 1970; 1980; White & may also produce mountains. This seems to be McKenzie 1989). illustrated by the Alps. Hence, differences in It is clear from these contributions that orogenic structures may be a question of geodynamic concepts, modelling and new tech- duration as much as a classification problem. niques such as deep seismic profiling and Did orogenic processes evolve and change isotopic methods are important but must always through time? Weaker bulk rheology of litho- be combined with field observations. Such spheric plates was probably responsible for the interdisciplinary work defines the new stage in wider distribution of intraplate deformation in orogenic studies which has succeeded the plate Archaean and Lower Proterozoic times. The tectonic revolution. It is also clear that abundance of anatectic granites in Archaean orogenesis is not only equated with the Wilson and Early Proterozoic orogens may have been cycle (where a subduction orogen may precede due to the large width/length ratio of obduction, followed by full closure of the these orogens. As demonstrated in younger and collision), but can also occur within con- orogens, horizontal heat transfer leading to tinental plates. in the thickened crust is more efficient Accretion that results from a collage of island in wider belts (Gaudemer et al. 1988). There is arcs and accretionary prisms juxtaposed to an no reason for excluding orogenesis at plate active continental margin (Irving et al. 1980; boundaries during history. Younger Ben-Avraham et al. 1981) also illustrates non- orogens probably evolved over longer periods Wilson-cycle orogenesis. Large horizontal strike and formed narrower belts along longer plate slip between geological units (suspect terranes) margins. The larger scale of tectonic plates in does not produce regional thickening. 'Orogeny' the Phanerozoic thus led to orogenic belts of must result from the partitioned compressional more dramatic dimensions. However, this dif- component of oblique convergence. ference in scale probably had little effect on the The inference from both observation and major geodynamic processes involved in modelling is that there must be subduction orogenesis. involved in orogens formed at plate boundaries. Therefore, these orogenic structures are funda- We thank the participants of the 'Orogeny through mentally asymmetric, usually synthetic with the time' seminar series in 1994 and our colleagues at ETH subducting plate. The two contrasting subduc- for stimulating discussions on orogenesis. We also tion types, A-type for continental subduction thank N. Kusznir for helpful discussion on the topic. and B-type for oceanic subduction (Oxburgh Reviewers are thanked for their contributions. 1972) may control, for understandable reasons of buoyancy, two types and/or stages of References orogenesis (e.g. Tibet and Oman respectively). ANDERSEN, T. B., JAMTVEIT, B., DEWEY, J. F. & Theoretically, the continental lithosphere is not SWENSSON, E. 1991. Subduction and eduction of dense enough to be significantly subducted into continental crust : major mechanisms during the mantle. However, geological restorations -continent collision and orogenic ex- Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

14 J.-P. BURG & M. FORD

tensional collapse, a model based on the south first record and some consequences. Contribution Norwegian Caledonides. Terra Nova 3,303-310. to and , 86, 107-118. ARGAND, E. 1924. La tectonique de l'Asie. Compte CHOUKROUNE, P. & ECORS PYRENEES TEAM 1990. Rendu de la 13~me Congres G~ologique Inter- The ECORS Pyrenean deep seismic profile. national, Bruxelles, 1,171-372. Reflection data and the overall structure of an ARTVUSHKOV, E. V. 1973. Stresses in the lithosphere . Tectonics, 8, 23-39. caused by crustal thickness inhomogeneities. CLOETINGH, S., WORTEL, R. & VLAAR, N. J. 1989. On Journal of Geophysical Research, 78, 7675-7708. the initiation of subduction zones. Pure and BEAUMONT, C. t~ QUINLAN, G. 1994. A geodynamic Applied , 129, 7-25. framework for interpreting crustal-scale seismic- CLOOS, M. 1982. Flow melanges: Numerical modeling reflectivity patterns in compressional orogens. and geologic constraints on their origin in the Geophysical Journal International, 116,754-783. Franciscan subduction complex, California. Geo- BEN-AVRAHAM, Z., NUR, A., JONES, D. & Cox, A. logical Society of America Bulletin, 93,330-345. 1981. Continental accretion and orogeny : From COBBOLD, P. R. & JACKSON, M. P. A. 1992. Gum resin oceanic to allochthonous terranes. Sci- (colophony): a suitable material for thermome- ence, 213, 47-54. chanical modelling of the lithosphere. Tectono- BIOT, M. A. 1961. Theory of folding of stratified , 210,255-271. viscoelastic media and its implications in tectonics --, DAVY, P., GAPAIS, D., ROSSELLO, E. A., and orogenesis. Geological Society of America SADYBAKASOV,E., THOMAS, J. C., TONDJI BIYo, J. Bulletin, 72, 1595-1620. J. • DE URREIZTIETA, M. 1993. Sedimentary BIRD, P. 1991. Lateral extrusion of lower crust from basins and crustal thickening. Sedimentary Geo- under high topography, in the isostatic limit. logy, 86, 77-89. Journal of Geophysical Research, 96, 10,275- COLEMAN, R. G. & WANG, X. 1995. Overview of the 10,286. geology and tectonics of UHPM. In: COLEMAN, BRUNEL, M. 1986. Ductile thrusting in the Himalayas: R. G. & WANG, X. (eds) Ultrahigh pressure shear sense criteria and stretching lineations. metamorphism. Cambridge University Press, Tectonics, 5, 2,247-265. Cambridge, 1-32. & KII~NAST,J.-R. 1986. Etude p6tro-structurale CONDIE, K. C. 1982. Plate tectonics and crustal des chevauchements ductiles himalayens sur la evolution. Pergamon Press. transversale de l'Everest-Makalu (N6pal orien- CONEY, P. J. 1970. The geotectonic cycle and the new tal). Canadian Journal of Earth Sciences, 23, global tectonics. Geological Society of America 1117-1137. Bulletin, 81,739-748. BURG, J.-P. & CHEN, G. M. 1984. Tectonics and -- 1987. The regional tectonic setting and possible structural zonation of southern Tibet, China. causes of Cenozoic extension in the North Nature 311,219-223. American Cordillera. Geological Society Special , DAVY, P. & MARTINOD, J. 1994a. Shortening of Publication, 28, 177-186. analogue models of the continental lithosphere: COOK, F. A. & VARSEK, J. L. 1994. Orogen-scale New hypothesis for the formation of the Tibetan d6collements. Review of Geophysics, 32, 37-60. . Tectonics, 13,475-483. COWEN, D. D. & SILLING, R. M. 1978. A dynamic , VAN DEN DRIESSCHE, J. & BRUN, J.-P. 1994b. scaled model of accretion at and its Syn- to post-thickening extension in the Variscan implications for the tectonic evolution of subduc- Belt of Western : Mode and structural tion complexes. Journal of Geophysical Research, consequences. G~ologie de la France, 3, 33-51. 83, 5389-5396. BUROV, E. B. & DIAMENT, M. 1995. The effective DAHLEN, F. A. 1984. Non-cohesive critical Coulomb elastic thickness (Te) of continental lithosphere: wedges: an exact solution. Journal of Geophysical What does it really mean? Journal of Geophysical Research, 89, 10,125-10,133. Research, 100, 3905-3927. --, SUPPE, J. & DAVIS, D. 1984. Mechanics of CADY, W. M. 1950. Classification of geotectonic fold-and-thrust belts and accretionary wedges: elements. American Geophysical Union Trans- Cohesive Coulomb theory. Journal of Geophysi- actions, 31,780-785. cal Research, 89, 10,087-10,101. CARTER, N. L. & TSENN, M. C. 1987. Flow properties DAVIS, D., SuPP~, J. & DAHLEN, F. A. 1983. of continental lithosphere. 136, Mechanics of fold-and-thrust belts and accretion- 27-63. ary wedges. Journal of Geophysical Research, 88, CHAPPLE, W. M. 1978. Mechanics of thin-skinned 1153-1172. fold-and-thrust belts. Geological Society of DAVY, P. & COBBOLD, P. 1991. Experiments on America Bulletin, 89, 1189-1198. shortening of a 4-layer model of the continental CHEMENDA, A. I., MATTAUER, M., MALAVIEILLE,J. t~ lithosphere. Tectonophysics, 188, 1-25. BOKUN, A. N. 1995. A mechanism for syn- -- & 1988. Indentation tectonics in nature and collisional rock exhumation and associated nor- experiment, 1. Experiments scaled for gravity. mal faulting: Results from physical modelling. Bulletin of the Geological Institute, University of Earth and Planetary Sciences Letters, 132, 225- Uppsala, New Series, 14, 129-141. 232. DENIEL, C., VIDAL, P., FERNANDEZ,A., LE FORT, P. (~ CHOPIN, C. 1984. Coesite and pure pyrope in high- PEUCAT, J.-J. 1987. Isotopic study of the Manaslu grade pelitic of the Western Alps: a granite (Himalaya, Nepal); inference on the age Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 15

and source of Himalayan . Contri- collision belts. Earth and butions to Mineralogy and Petrology, 96, 78-92. Letters, 89, 48-62. DENNIS, J. G. 1967. International tectonic dictionary. GILBERT, G. K. 1890. Lake Bonneville. US Geological American Association of Geologists Survey Monographs, 1. Memoirs, 7. GILLULY, J. 1973. Steady plate motions and episodic DEPAOLO, D. J. 1981. Nd isotope studies; Some new orogeny and . Geological Society of perspectives on and evolution. America Bulletin, 84,499-514. LOS Transaction of the American Geophysical GOLEBY, B. R., KENNETr, B. L. N., WRIGHT, C., Union, 62, 137-140. SHAW, R. D. • LAMBECK, K. 1990. Seismic DERCOURT, J., RICOU, L.-E. & VRIELYNCK, B. 1993. reflection profiling in the Proterozoic Arunta Atlas Tethys palaeoenvironmental maps. Gau- Block, central Australia: processing for testing thier-Villars. models of tectonic evolution. Tectonophysics, DEWEY, J. F. 1988. Extensional collapse of orogens. 173,257-268. Tectonics, 7, 1123-1139. HARGRAVES, R. B. 1976. Precambrian geologic his- & BIRD, J. M. 1970. Mountain belts and the new tory. Science, 193,363-371. global tectonics. Journal of Geophysical Re- HARLEY, S. L. & CARSWELL, D. A. 1995. Ultradeep search, 75, 2625-2647. crustal metamorphism: A prospective view. Jour- & BURKE, K. 1973. Tibetan, Variscan and nal of Geophysical Research, 100, 8367-8380. Precambrian basement reactivation: Products of HIRN, A. 1988. Features of the crust-mantle structure . Journal of Geology, 81, of Himalayas-Tibet: a comparison with seismic 683-692. traverses of Alpine, Pyrenean and Variscan DICKINSON, W. R. 1971. Plate tectonic models of orogenic belts. Philosophical Transactions of the geosynclines. Earth and Planetary Sciences Royal Society of London, A 326, 17-32. Letters, 10,165-174. HOEFMAN, P. F. 1988. United plates of America, the DIETZ, R. S. 1972. Geosynclines, mountains and birth of a craton: early Proterozoic assembly and continent-building. Scientific American, 226, growth of . Annual Review of Earth and 30-38. Planetary Sciences, 16,543-603. EMERMAN, S. H. & TURCOTTE, D. L. 1983. A fluid 1989. Precambrian geology and tectonic history model for the shape of accretionary prisms. Earth of . In: BALLY, A. B. & PALMER, and Planetary Sciences Letters, 56,387-397. A. R. (eds) The geology of North America; An ENGLAND, P. 1992. Deformation of the continental overview. Geological Society of America, Boul- crust. In: BROWN, G., HAWKESWORTH, C. & der, 447-512. WILSON, C. (eds) Understanding the Earth. A new ~, TIRRUL, R., KING, J. E., ST-ONGE, M. R. & synthesis. Cambridge University Press, Cam- LUCAS, S. B. 1988. Axial projections and modes bridge, 275-300. of crustal thickening, eastern Wopmay orogen, & HOLLAND, T. J. B. 1979. Archimedes and the northwest Canadian . In: CLARK, S. P. JR Tauern eclogites: The role of buoyancy in the (ed.) Processes in Continental Lithospheric De- preservation of exotic eclogite blocks. Earth and formation. Geological Society of America, Planetary Sc&nces Letters, 44,287-294. Special Papers, 218, 1-29. 8£ HOUSEMAN, G. A. 1986. Finite strain calcu- HOUSEMAN, G. A. t~z ENGLAND, P. C. 1986. Finite lations of continental deformation 2. Comparison strain calculations of continental deformation 1. with the India-Asia collision zone. Journal of Method and general results for convergent zones. Geophysical Research, 91, 3664-3676. Journal of Geophysical Research, 91, 3651-3663. & 1989. Extension during continental --, MCKENZIE, D. P. & MOLNAR, P. 1981. Convec- convergence, with application to the Tibetan tive instability of a thickened boundary layer and plateau. Journal of Geophysical Research, 94, its relevance for the thermal evolution of con- 17,561-17,579. tinental convergent belts. Journal of Geophysical & MCKENZIE, D. P. 1983. Correction to: A thin Research, 86, 6115-6132. viscous sheet model for continental deformation. IRVING, E. J., MONGER, W. H. & YOLE, R. W. 1980. Geophysical Journal of the Royal Astronomical New paleomagnetic evidence for displaced ter- Society, 73,523-532. fanes in British Columbia. Special Papers of the & MOLNAR, P. 1993. Cause and effect among Geological Association of Canada, 20,441-456. thrust and normal faulting, anatectic melting and ISACKS, B., OLIVER, J. t~ SYKES, L. R. 1968. Seismo- exhumation in the Himalaya. In: TRELOAR,P. J. & logy and the new global tectonics. Journal of SEARLE, N. P. (eds) qv. 401-411. Geophysical Research, 73, 5855-5899. FOWLER, C. M. R. 1990. The . Cambridge KARIG, D. E., BARBER, A. J., CHARLTON, T. R., University Press. KLEMPERER, S. ~ HUSSONG, D. M. 1987. Nature GANSSER, A. 1964. Geology of the Himalayas. Inter- and distribution of deformation across the banda- science. Arc-Australian collision zone at Timor. Geologi- 1983. Geology of the Bhutan Himalaya. cal Society of America Bulletin, 98, 18-32. Denkschriften der Schweizerischen Naturfor- KIRBY, S. H. 1985. observations schenden Gesellschaft, 96, 1-181. pertinent to the rheology of the continental GAUDEMER, Y., JAUPART, C. & TAPPONNIER, P. 1988. lithosphere and the localization of strain along Thermal control on post-orogenic extension in shear zones. Tectonophysics, 119, 1-27. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

16 J.-P. BURG & M. FORD

KOBER, L. 1928. Der Bau der Erde. Zweite NIKISHIN, A. M., CLOETINGH, S., LOBKOVSKY, L. I., Auflage. Borntraeger. BUROV, E. B. & LANKREIJER, A. C. 1993. KRONER, A. 1981. Precambrian plate tectonics. In: Continental lithosphere folding in Central Asia KRONER, A. (ed.) Precambrian plate tectonics. (Part I): constraints from geological observations. Elsevier, Amsterdam, 56-90. Tectonophysics, 226, 59-72. KUSZNIR, N. & KARNER, G. 1985. Dependence of the OXBURGH, E. R. 1972. Flake tectonics and continental flexural rigidity of the continental lithosphere on collision. Nature, 239,202-215. theology and temperature. Nature, 316, 139-142. PANZA, G. F. & MUELLER,S. 1979. The plate boundary 8£ PARK, R. G. 1984. Intraplate lithosphere between Eurasia and in the Alpine area. strength and heat flow. Geophysical Journal of the Memorie de Scienze Geologiche, 33, 43-50. Royal Astronomical Society, 79, 513-538. PARSONS, B. & MCKENZIE,D. 1978. LAMBECK, K. 1983. Structure and evolution of the and the thermal structure of the plates. Journal of intracratonic basins of central Australia. Geo- Geophysical Research, 83, 4485-4496. physical Journal of the Royal Astronomical So- PFIFFNER, O. A., LEHNER, P., HEITZMANN, P., ciety, 74,843-886. MUELLER, S. • STECK, A. (eds) 1996. Deep LAUBSCHER, H. 1974. The tectonics of subduction in structure of the Swiss Alps - results of the National the Alpine system. Memoire delta Societ~ geolo- Research Program 20 (NFP20). Birkh/iuser, gica Italiana 13, suppl. 2, 275-283. Basel. LE FORT, P. 1975. The collided range. Present PLATT, J. P. 1986. Dynamics of orogenic wedges and the knowledge of the . American uplift of high-pressure metamorphic rocks. Geo- Journal of Science, 275-A, 1--44. logical Society of America Bulletin, 97,1037-1053. LE PICHON, X. 1968. Sea-floor spreading and continen- 1993. Exhumation of high-pressure rocks: a tal drift. Journal of Geophysical Research, 73, review of concepts and processes. Terra Nova, 5, 3661-3697. 119-133. [VIARTINOD, J. & DAVY, P. 1992. Periodic instabilities -- & ENGLAND, P. C. 1994. Convective removal of during compression or extension of the litho- lithosphere beneath mountain belts: thermal and sphere 1. Deformation modes from an analytical mechanical consequences. American Journal of perturbation method. Journal of Geophysical Science, 294,307-336. Research, 97, 1999-2014. RAMBERG, H. 1970. Folding of laterally compressed -- & -- 1994. Periodic instabilities during com- multilayers in the field of gravity, II, Numerical pression of the lithosphere 2. Analogue experi- examples. Physics of the Earth and Planetary ments. Journal of Geophysical Research, 99, Interiors 4, 83-120. 12,057-12,069. RANALLI, G. 1995. Rheology of the Earth. Chapman & MCKENZIE, D. P. & PARKER, R. L. 1967. The North Hail. Pacific: an example of tectonics on a sphere. & MURPHY, D. C. 1987. Rheological stratification Nature, 216, 1276-1280. of the lithosphere. Tectonophysics, 132,281-295. MEISSNER, R. 1989. Rupture, creep lamellae and REED, J. C. J., BALL, T. T., LANG FARMER, G. & crocodiles: happenings in the continental crust. HAMILTON, W. B. 1993. A broader view. In: REED, Terra Nova, 1, 17-28. J. C. JR., BICKFORD,M. E., HOUSTON, R. S., LINK, MITCHELL, A. H. & READING,H. G. 1969. Continental P. K., RANKIN, D. W., SIMS, P. K., VAN SCHMUS, margins, geosynclines and ocean floor spreading. W. R. (eds) The Geology of North America. Journal of Geology, 77,629-646. Precambrian: Conterminous US. Geological So- MrVASHIRO, A., AKI, K. & SENG6R, A. M. C. 1982. ciety of America, , 597-636. Orogeny. John Wiley & Sons. ROBERTSON, A. H. F., SEARLE, M. P. & RIES, A. C. MOLNAR, P. & GRAY, D. 1979. Subduction of (eds) 1990. The geology and tectonics of the Oman continental lithosphere: Some constraints and . Geological Society, London, Special uncertainties. Geology, 7, 58-62. Publications, 49. -- & TAPPONNIER, P. 1975. Cenozoic tectonics of ROURE, F., HEITZMANN, P. & POLINO, R. (eds) 1990. Asia: Effects of a continental collision. Science, Deep structure of the Alps, MOmoires de la Soci6t6 189,419-426. g6ologique de Suisse, 1, Ziirich. , BURCHFIEL, B. C., LIAN6, K. & ZHAO, Z. 1987. SANDIFORD, M. 1989. Horizontal structures in granu- Geomorphic evidence for active faulting in the lite terrains: A record of mountain building or Altyn Tagh and northern Tibet and qualitative mountain collapse? Geology 17,449-452. estimates of its contribution to the convergence of SCHMID, S. M., FROITZHEIM, N., PFIFFNER, A., India and Eurasia. Geology, 15,249-253. SCHONBORN, G. & KISSLING, E. 1996. Geo- , ENGLAND, P. & MARTINOD, J. 1993. Mantle physical-geological transect and tectonic evol- dynamics, uplift of the , and the ution of the Swiss-Italian Alps. Tectonics, in Indian monsoon. Review of Geophysics, 31, press. 357-396. SCHREYER, W. 1995. Ultradeep metamorphic rocks: MORGAN, W. J. 1968. Rises, trenches, great faults, and The retrospective viewpoint. Journal of Geo- crustal blocks. Journal of Geophysical Research, physical Research, 100, 8353-8366. 73, 1959-1982. SEBER, D., BARAZANGI, M., IBENBRAHIM, A. & NICOLAS, A. 1990. Les montagnes sous la met. DEMNATI, A. 1996. Geophysical evidence for Editions du BRGM. lithospheric delamination beneath the Alboran Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

OVERVIEW 17

Sea and Rif-Betic mountains. Nature, 379, 785- and an initial stiff inclusion. Geophysical Journal 790. of the Royal Astronomical Society, 84, 279-310. SHEMENOA, A. I. & GROCnOLSKY, A. L. 1992. Physical VON HtmNE, R. & LEE, H. 1983. The possible modelling of lithosphere subduction in collision significance of pore fluid pressures in subduction zones. Tectonophysics, 216,273-290. zones. In: WATKrNS, J. S. & DIt~,CE, C. L. (eds) SLEEP, N. H. & WINDLEY, B. F. 1982. Archean plate Continential Margin Geology. American Associ- tectonics; Constraints and inferences. Journal of ation of Petroleum Geologists Memoirs, 34, Geology, 90,363-379. 781-791. STEPHENSON, R. A. & CLOETINGH, S. A. P. L. 1991. WEBER, K. 1984. Variation in tectonic style with time Some examples and mechanical aspects of con- (Variscan and Proterozoic systems). In: HOL- tinental lithospheric folding. Tectonophysics, LAND, H. D. & TRENDALL,A. F. (eds) Patterns of 188, 27-37. change in Earth evolution. Springer-Verlag, Ber- , RICKETTS, B. D., CLOETINGH, S. A. P. L. & lin, 371-386. BEEKMAN, F. 1990. Lithosphere folds in the WEGEN~R, A. 1912. Die Entstehung der Kontinente. Eurekan Orogen, Arctic Canada? Geology, 18, Geologische Rundschau, 3,276-292. 103-106. WELLS, P. R. A. 1980. Thermal models for the TAPPONNIER, P. 1977. Evolution tectonique du sys- magmatic accretion and subsequent metamor- t6me alpin en M6diterran6e: poinqonnement et phism of continental crust. Earth and Planetary 6crasement rigide-plastique. Bulletin de la Soci~t~ Science Letters, 46,253-265. g~ologique de France, 7-19,437-460. WERNICKE, B. 1981. Low-angle normal faults in the & MOLNAR, P. 1976. Slip-line field theory and Basin and Range Province: nappe tectonics in an large-scale continental tectonics. Nature, 264, extending orogen. Nature, 291,645--648. 319-324. WHITE, R. S. & MCKENZlE, D. P. 1989. Magmatism at , PELTZER, G., LE DAIN, A. Y., ARMIJO, R. & rift zones: The generation of volcanic continental COBBOLD, P. 1982. Propagating extrusion tec- margins and . Journal of Geophysical tonics in Asia: New insights from simple experi- Research, 94, 7685-7729. ments with plasticine. Geology, 10, 611-616. WILLETT, S., BEAUMONT, C. & FULLSACK, P. 1993. TRELOAR, P. J. & SEARLE, M. P. (eds) 1993. Himalayan Mechanical model for the tectonics of doubly Tectonics. Geological Society, London, Special vergent compressional orogens. Geology, 21, Publications 74. 371-374. TURCOTrE, D. L. & SCHUBERT, G. 1982. Geodynamics: WTLSON, J. T. 1966. Did the Atlantic close and then applications of continuum physics to geological reopen? Nature, 211,676-681. problems. John Wiley & Sons. WYNNE-EDWARDS, H. R. 1976. Proterozoic ensialic VALASEK, P. 1992. The tectonic structure of the Swiss orogenesis: the millipede model of ductile plate Alpine crust interpreted from a 2D network of deep tectonics. American Journal of Science, 276, crustal seismic profiles and an evaluation of 3D 927-953. effects. PhD, ETH, Ziirich. ZWART, H. J. 1967. The duality of orogenic belts. VILOT]TE, J. P., MADARIAGA, R., DAIGNI~RES, M. & Geologie en Mijnbouw, 46,283-309. ZINKIEWICZ, O. 1986. Numerical study of con- tinental collision: influence of buoyancy forces