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Collision Orogeny Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 PROCESSES OF COLLISION OROGENY Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 Shortening of continental lithosphere: the neotectonics of Eastern Anatolia a young collision zone J.F. Dewey, M.R. Hempton, W.S.F. Kidd, F. Saroglu & A.M.C. ~eng6r SUMMARY: We use the tectonics of Eastern Anatolia to exemplify many of the different aspects of collision tectonics, namely the formation of plateaux, thrust belts, foreland flexures, widespread foreland/hinterland deformation zones and orogenic collapse/distension zones. Eastern Anatolia is a 2 km high plateau bounded to the S by the southward-verging Bitlis Thrust Zone and to the N by the Pontide/Minor Caucasus Zone. It has developed as the surface expression of a zone of progressively thickening crust beginning about 12 Ma in the medial Miocene and has resulted from the squeezing and shortening of Eastern Anatolia between the Arabian and European Plates following the Serravallian demise of the last oceanic or quasi- oceanic tract between Arabia and Eurasia. Thickening of the crust to about 52 km has been accompanied by major strike-slip faulting on the rightqateral N Anatolian Transform Fault (NATF) and the left-lateral E Anatolian Transform Fault (EATF) which approximately bound an Anatolian Wedge that is being driven westwards to override the oceanic lithosphere of the Mediterranean along subduction zones from Cephalonia to Crete, and Rhodes to Cyprus. This neotectonic regime began about 12 Ma in Late Serravallian times with uplift from wide- spread littoral/neritic marine conditions to open seasonal wooded savanna with coiluvial, fluvial and limnic environments, and the deposition of the thick Tortonian Kythrean Flysch in the Eastern Mediterranean. Earthquake hypocentres are scattered throughout the region but large earthquakes are concentrated mainly on the major faults and are mostly shallow, supporting the idea of a brittle elastic lid with hypocentres concentrated towards its base with more ductile deformation in the middle and lower crust. Neotectonic magmatic suites are nepheline- hypersthene normative alkali basalts of mantle origin, and silicic/intermediate/mafic calc- alkaline suites, both suites occurring in pull-apart basins in strike-slip regimes and along N-S extensional fissures, and both suites showing a strong change to central activity in the Pliocene. Upper-crustal strains appear to be discontinuous in space and time, with zones of strong shortening representing shoaling of crustal detachment zones flattening between 5 and 10 km. Approximately NW- (dextral) and NE- (sinistral) trending lineaments bound less deformed wedges (low relief seismically 'dead' areas) and vary from simple strike-slip faults to com- plicated braided transform-flake boundaries with pull-apart and compressional segments (N and E Anatolian Transform Faults). Volcanoes lie in grabens on N-S 'cracks' that extend into the Arabian Foreland and in transcurrent pull-aparts. Major extensional basins lie at plate (Adana) and flake (Karliova) triple junctions and result from compatibility problems. Apart from impacts and possible but difficult Continental collision involves the progres- to evaluate sub-lithospheric influences, the sive impingement of buoyant or highstanding geology of the continental crust is the con- terranes with subduction zones. All scales and sequence of the flexure, stretching and variations exist on this theme between the shortening of the lithosphere. Rapid stretching collision of seamounts and seamount chains and shortening of the lithosphere produce with arcs through the collision of oceanic isothermal thinning and thickening respec- plateaux and microcontinents with arcs to the tively, with consequent basins and moun- collision of large continental masses. The scale tains. Thermal relaxation generates further of collision dictates the. style, duration and subsidence or uplift, enhanced, respectively, intensity of the resulting strain systems and by sedimentation and denudation. Conse- sequences (Dewey 1977). Colliding continental quently, most vertical motions leading to all margins are irregular and strain sequences are the subtleties and complexities of strati- usually diachronous along great strike lengths graphic development are the result of litho- along suture zones (Dewey & Burke 1973). spheric deformation. Continental collision is Prior to terminal continental collision, one or one of the principal mechanisms leading to both continental margins may have had a long lithospheric/crustal thickening and mountain and complex history of exotic terrane assembly building and is an appropriate topic for a (Coney et el., 1980). William Smith thematic meeting honouring Continental convergent plate boundaries Robert Shackleton, that prime observer and such as the Alpine/Himalayan System (Fig. 1), interpreter of rocks in continental deformation are wide, diffuse and complicated zones where zones. relative plate displacements are converted into From COWARD, M. P. & RtES, A. C. (eds), 1986, Collision Tectonics, Geological Society Special Publication No. 19, pp. 3-36. Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 4 J.F. Dewey et al. <. .=, r-i r., E~.. ~ E t-- © © ©:.~ ~ ~ ~.~,~ ~-_0 >- _= 0=h: = > | • . "~ .~ ..o | 0 X 0 "~ .~ ,-.,.- -~ ..~ .,-, "-' ~ ~_ ~ ~ ~[.-- ~_o..o" " Z o,= o~ E ~: .=. = , ~ I;;;; ¢~ Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 Neotectonics of E Anatolia 5 complex and variable strains and smaller the thickened crust at an Argand Number block-bounding displacements. This contrasts (ratio of stress caused by crustal thickness dif- with oceanic plate boundaries, which are ference and stress needed to continue con- generally narrow, relatively simple zones in vergence, England & McKenzie 1982) of 3. which only a small portion of relative plate This model has the particular merit of explain- motion is converted into strain and smaller dis- ing E-W lateral wedging and extension as a placements (McKenzie 1972). This contrast is late stage consequence of crustal thickening. probably due to the relative weakness and The Tibetan Plateau is the higher and larger buoyancy of quartz and the relative strength of the two major plateaux in the Alpine/ and negative buoyancy of olivine as the prin- Himalayan system (Fig. 1), the other being the cipal mineral phases in the continents and Turkish/Iranian Plateau, a zone of lithospheric oceans respectively. Also, the great inhomo- horizontal shortening about 2 km above sea geneity and anisotropy of the continental crust, level ($eng6r & Kidd 1979). Such plateaux, riddled with zones of low strength, generated with roughly constant mean elevation, are one and modified by many varied mechanisms, of five tectonic components in collisional contrasts with the relative homogeneity of the systems (Figs 1 and 2), namely plateaux, thrust oceanic lithosphere generated by plate accre- belts, foreland lithospheric flexures, wide- tion with. fracture zone modifications (Dewey spread foreland/hinterland deformation zones 1982). and orogenic collapse/distension zones. We A basic problem of collisional tectonics is here define foreland and hinterland to mean how relative plate displacement directions and those regions exterior to the outermost major rates are converted in strains and strain rates overthrust belts in the direction and away from within the convergent plate boundary zone the direction, respectively, of principal oroge- (Fig. 1). Our understanding of this problem, nic vergence. Thrust belts and foreland flexures although incomplete, has progressed greatly are common to all collisional systems, where- since Argand (1924) first explained the as plateaux, widespread foreland/hinterland Himalayan Orogen as a result of simple under- deformation and collapse zones may or may thrusting of India beneath Asia, a mechanism not be present in a particular portion of the advanced today by Powell & Conaghan (1973) orogen. Barazangi & Ni (1982), Ni & York (1978) and Thrust belts develop principally where the Ni & Barazangi (1984), among others, to thinned continental crust of a rifted margin is explain the thick Tibetan crust (Chen & progressively restacked and thickened toward Molnar 1981; Molnar & Chen 1983). Two the foreland. This commonly involves thrust further competing models have been suggested rejuvenation of old listric normal faults to explain the Tibetan Plateau (Fig. 1). Molnar (Jackson 1980) and thrust shortening is usually & Tapponnier (1975, 1977a,b, 1978, 1979, initially below sea level before the crust is 1981), Tapponnier & Molnar (1976, 1977) and restacked to 30 km. The oldest, highest, Tapponnier et al (1981, 1982) have advanced a internal, basement-cored nappes are generally horizontal plane strain slip-line solution to discontinuous along orogenic strike, whereas explain the pattern of strike-slip wedging and younger, exterior foreland thinner-skinned E-W extension in Tibet generated by penin- fold-thrust belts are continuous and highly sular India behaving as a rigid indenter. cylindroidal (Fig. 3). Where detachment Horizontal plane strain alone, however, cannot occurs along a weak horizon within a foreland explain the thickened Tibetan crust (> 80 km). sequence, rocks above the detachment can An alternative view is that the Tibetan litho- shorten significantly independently of the base-
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