Structural evolution of the Vardar root zone, northern Greece

JAY ZIMMERMAN, JR. * Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, New York 11790 JOHN V. ROSS Department of Geology, University of British Columbia, Vancouver 8, British Columbia

ABSTRACT characteristics. Our investigation suggests designated upper—middle Eocene by Mer- that a root zone has been preserved in the cier (1973) on the basis of spore and pollen Allochthonous alpine ophiolites in eastern Vardar zone. correlation of a single tuffaceous unit that is northern Greece are related to destruction metamorphically and structurally similar to of an ocean basin during Late Cretaceous General Geology of the Vardar Zone Mesozoic rocks in the same area. This is time. Associated plate interaction caused considered to represent a dynamothermal regional-scale flexure of the internal The Vardar zone in Greece (Fig. 1) is a event of local significance. In general, re- portions of obducted thrust sheets com- north-northwest-trending deformed belt gional metamorphism appears to intensify posed of continental and oceanic rocks. approximately 60 km wide, bounded on the from southwest to northeast across north- This flexure, the Vardar root zone, is northeast and southwest by the Rhodopian central Greece. Late Cretaceous carbonates characterized by at least four successive unmetamorphosed in the sub-Pelagonian phases of mesoscopic folding accompanied zone to the southwest (Zimmerman, 1972) by development of mylonites and rotation have been metamorphosed to greenschist of s surfaces from a subhorizontal to a ver- grade in the Vardar zone (Mercier, 1973). tical or near-vertical attitude. Earlier fold Deformation is most intense in the east- sets (F, and F2) in a number of lithologic ern part of the zone. Pre-Mesozoic base- units have been rotated and flattened in the ment, Mesozoic, and, possibly, early Ter- core of the root flexure during its formation tiary rocks have been juxtaposed along (F3), which suggests that this deformation thrust faults and are typically discontinuous phase represents a single tectonic event. along strike. The eastern part of the Vardar Subsequent collision of Rhodopian zone zone is dominated by a breached anticlinal basement with the root zone may have flexure of regional extent having a vertical- caused renewed southwest-directed thrust- to-overturned, steeply southwestward- ing and F4 kink folds. The Olympus win- dipping eastern limb and a gently dow through late Paleozoic Pelagonian southwestward-dipping western limb. This basement rocks into Eocene flysch suggests structure has been designated the Vardar that the Vardar root zone and other major root zone (Ross and Zimmerman, 1973). tectonic units in north-central Greece may Deformation increases from west to east be allochthonous. across the Vardar zone, becoming most in- tense toward the core and overturned east- INTRODUCTION ern limb of the root-zone flexure. This area Figure 1. Generalized map of Greece showing is characterized by subvertical to vertical s major isopic zones and the location of the Olym- surfaces, including lithologic and tectonic The presence of allochthonous alpine- pus window (after Aubouin, 1965). type ultramafic rocks in the central Hel- contacts, foliation, and axial surfaces of lenides has been attributed to processes that mesoscopic folds; the development of involve destruction of small ocean basins and Pelagonian basement massifs, respec- axial-plane cleavage; severely flattened accompanied by supracontinental tectonic tively (Aubouin, 1965). Rocks within the polyphase folds; and locally abundant emplacement of oceanic crust and upper zone consist predominantly of deformed mylonite. mantle (Temple and Zimmerman, 1969; Mesozoic shelf carbonates, shale, gray- Dercourt, 1972; Boccaletti and others, wacke, and volcaniclastics, metamorphosed Mesoscopic Structures in the 1974). Proponents of a single late Mesozoic to greenschist facies. Locally serpentinized Vardar Root Zone subduction zone (Bernoulli and Laubscher, ultramafites and associated mafic rocks, 1972; Zimmerman, 1972) as opposed to typically noritic gabbro, are common, and Structural investigation was concentrated multiple areas of subduction (Hynes and the zone can be regarded as the general in the central and eastern Vardar zone others, 1972) suggested the Vardar zone in northeastward limit of abundant ophiolite along the western limb and in the core of north-central Greece as the site of subduct- distribution in northern Greece. Crystalline the root flexure. In this area, four superim- ive activity. One orogenic model based rocks representing pre-Mesozoic basement posed phases of folding are recognized. upon plate convergence results in the de- have been tectonically incorporated into the Phase 1 (FJ. F, structures are small, velopment of a root zone (Roeder, 1973), a zone. This material has undergone retro- nearly isoclinal folds and are the oldest complex anticlinal structure of regional ex- grade metamorphism from above garnet penetrative folds observed (Figs. 2, 3). Orig- tent having predictable and identifiable isograd to the greenschist grade typical of inal vergence was toward the west or mesoscopic and macroscopic structural adjacent Mesozoic units. The dominant southwest, with axial surfaces dipping greenschist-facies regional metamorphism gently eastward. Where observed, F, is al- that characterizes the Vardar zone is con- ways refolded by F2. * Present address: Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois sidered to be Late Cretaceous (Mercier, Phase 2 (F2). F2 is also represented by 62901. 1973). A younger metamorphic event was small, mesoscopic folds whose original ver-

Geological Society of America Bulletin, v. 87, p. 1547-1550, 6 figs., November 1976, Doc. no. 61102. 1547

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Figure 2. Fj fold (dashed line = axial surface trace) refolded by F2 (solid line = axial surface trace) on western limb of the root-zone flexure. FL and F2 have not been strongly modified by F3 at this location. Pen for scale. View looking approxi- mately north. 4

Figure 3. Diagrammatic representa- tion of superposed folding in the Vardar root zone. A. F, verging west or slightly south of west. B. F, refolded by F2 which verges northwest. C. F, + F2 rotated and flattened by formation of the root-zone flexure (F3). See text for description of F4 structures. •

gence was probably northwestward, with Poles to F, axial surfaces folded by F2 northeast in the eroded core and steep limb axial surfaces dipping gently to the south- measured west of the hinge line of the of the root-zone flexure where F, + F2 have east. On the western limb of the root flex- root-zone flexure are distributed along a been rotated and flattened by F3 show a ure, folding is tight, but both phases can be diffuse girdle (TT2) that defines a much more regular geometric relationship distinguished (Fig. 2). southwestward-dipping surface (Figs. 5A, (Fig. 5B). A diffuse point maximum (1 AP3) Phase 3 (F3). F3 is the regional flexure 5B). A point maximum of poles to F2 axial representing a concentration of poles to Fj that dominates the Vardar root zone. It is surfaces (lAP2) is located in the northern + F2 deformed by F3 lies along the 7t3 girdle. an open, asymmetric, macroscopic, part of the net. A constructed F2 axial sur- A constructed average axial plane (AP3) and breached anticlinal structure with a west- face (AP2) and a fold axis (L2) for F2 are fold axis (L3) conform to the north- southwestward-dipping axial surface. In based on orthogonal relationships with AP2 northwest regional strike characteristic of the hinge area, F, and F2 have been rotated and 7r2. In a set of cylindrical folds that have major lithologic units in the eastern Vardar and flattened by F3 and are no longer dis- not been subsequently modified, L„ and AP„ zone. tinguishable as separate phases of folding should lie on AP„ and 7r„ circles, respective- From the sense of rotation of F4 kink (Figs. 3, 4). ly. That this orthogonal relationship does folds superimposed on earlier structures (F,

Phase 4 (F4). F4 is expressed by conju- not hold for F2 folding of Ft could be at- to F3), an approximate regional orientation - gate kink folds. One set strikes northeast tributed to one or both of the following: (1) of o ,, o-2, and cr3 (greatest, intermediate, and dips to the northwest. The other strikes presence of an undiscovered pre-F2 fold and least principal compressive stress) can northwest and dips southwestward (Fig. 4). phase and (2) effects of F3 on prior phases be inferred (Fig. 5C). CT] is subperpendicular Figures 5A, 5B, and 5C are lower hemi- even in locations removed from the hinge to the axial surface constructed from F3 sphere, equal-area projections of poles to area of the root flexure where rotation and data, which suggests that F4 represents a axial surfaces of mesoscopic folds in the flattening are not obvious. continuation of the stress configuration re- Vardar and eastern Pelagonian zones. Folds measured farther to the east and sponsible for F3 folding but under condi- tions of lower temperature and pressure, which resulted in a less ductile response in the same lithologic units. MM H Involvement of Paleozoic Rocks in Mesozoic Tectonic Events Figure 4. Rotated and flattened F! + F2 folds in the The distribution of poles to the axial sur- core of the root-zone flex- faces of prominent mesoscopic folds in ure (F3). Attenuated fold basement rocks, both in the Vardar zone hinges can be seen in the left and on the eastern flank of the Pelagonian and central parts of the massif, conforms to that of Mesozoic photograph. S surfaces are rocks in the Vardar zone and suggests that kinked by F4 that crosses basement was involved in at least phases F! the photograph from upper through F (Fig. 5A). Involvement of base- right to lower left. View 3 toward the northwest. Pen ment in Mesozoic deformation is further for scale. suggested by the following: 1. The attitudes of tabular sheets of basement rock are parallel to thrust and ro- tated slices of Mesozoic metasediments and ultramafic units in the eastern part of the root zone.

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Figure 5. A. Lower hemisphere, equal-area projection of 193 poles to axial surfaces of F, through F4 in Mesozoic metamorphic rocks of the Vardar zone (dots) and 46 poles to axial sur- faces of small mesoscopic folds in basement rocks of the Vardar and eastern Pelagonian zones (open circles). TT2 = girdle distribution of poles to F, refolded by F2. Tr3 = girdle of poles to F, + F2 rotated and flattened by F3. X AP2 and _L AP3 are point maxima of poles to F2 and F3, respectively. Curved arrows around F4 poles represent the sense of rotation of kink-fold sets in the Var- dar zone. B. Synoptic diagram of data distribu- Figure 6. Diagrammatic east-west cross sec- tion from A. AP2 and AP3 are inferred average tion (looking north) showing the suggested se- axial surface traces of F2 and F3, respectively. L2 quence of events leading to the formation of the and L3 represent inferred axes to F2 and F3 fold Vardar root zone (vrz) and the present juxtaposi- sets. L3 parallels regional strike in the Vardar tion of the Pelagonian (pz), Vardar, and Rhodo- zone. C. Inferred orientation of regional cr,, cr2, pian (rz) zones. Black area = continental (thick) and cr3 from orientation and rotation sense of F4 and oceanic (thin) crust. White areas = subcrus- kink folds. tal lithosphere. The possible allochthonous na- ture of the Pelagonian and Vardar zones suggested by the presence of the Olympus win- 2. Basement rocks of higher regional Figure 6A represents the inferred relative dow is not shown in this figure. Further explana- metamorphic grades have undergone positions of the Pelagonian (to the west) tion in text. Illustration adapted from Roeder diaphthoresis to greenschist-facies as- and Rhodopian continental blocks and an (1973). semblages typical of the prograde Mesozoic "east"-dipping subduction zone during an units. early obductive stage. Crust along one or folding superimposed on F,. As the regional 3. In the Pelagonian zone, local expo- both continental margins may have been at- flexure developed, the F, + F2 fold system sures of intensely recrystallized carbonate tenuated by earlier rifting. The leading was progressively flattened in the core area rock, probably correlative with Triassic (eastern) edge of the Pelagonian block has but was preserved with minimum modifica- carbonates in the Vardar zone and been partially subduced, and ultramafic and tion by F3 on the western limb. Triassic-Jurassic marble in the sub- associated mafic rocks, together with In Figure 6C, westward subduction con- Pelagonian zone to the southwest (Zim- spatially related marine trench sediments tinues until a continental margin represent- merman, 1972), have developed a strong and allochthonous slices of detached conti- ing the present Rhodopian zone is passively cleavage pattern not evident in the other nental crystalline rocks, have been thrust rafted into juxtaposition and locks against areas. These exposures may represent in- westward over shelf sediments and au- the root zone from the east. F4 structures liers of carbonate shelf cover folded along tochthonous continental basement. It is may have developed at this time or late in southwest-directed, basement-involved likely that F! folds were formed during this the third stage. Subduction can now be ex- thrust faults during late Mesozoic time. phase. pected to shift to the nearest mechanically 4. In the Mount Olympus area, God- Figure 6B shows formation of the root- feasible location. The extent to which in- friaux (1962) described a tectonic window zone flexure by the combined processes of teraction with the Rhodopian continental through allochthonous Pelagonian base- isostatic rebound of subduced continental edge may have modified the Vardar root ment into nonmetamorphosed Eocene lithosphere and a reversal of the direction zone has not been determined and must flysch. of subduction. Early in this stage, incipient wait until the structure of the Rhodopian flexing of the internal portions of the ob- zone east of Thessaloniki has been studied Regional Tectonic Model ducted thrust sheets may have produced in detail. frictional drag that inhibited tectonic trans- A model of the regional tectonic evolu- port to the west or southwest but allowed Significance of the Olympus Window tion of north-central Greece is given in Fig- continued thrusting in a northwesterly di- ure 6 (see Temple and Zimmerman, 1969; rection. This change in transport direction The Olympus window (Fig. 1) suggests Roeder, 1973). was accompanied by northwest-verging F2 southwestward transport of the principal

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lithologie and tectonic units of the Vardar, REFERENCES CITED matique des zones internes des Hellénides, Pelagonian, and sub-Pelagonian zones Vol. II: Annales Géol. Pays Helléniques, 1st ser., tome XX p. 597-792. (Zimmerman, 1972; Bernoulli and Aubouin, J., 1965, Geosynclines: New York, Roeder, D. H., 1973, Subduction and orogeny: Laubscher, 1972) and may be related to col- Elsevier, 335 p. Bernoulli, D., and Laubscher, H., 1972, The Jour. Geophys. Research, v. 78, p. 5005- lision of the Rhodopian massif. The win- 5024. dow is south of and approximately adjacent palinspastic problem of the Hellenides: Ec- logae Geol. Helvetiae, no. 65/1, p. 107— Ross, J. V., and Zimmerman, J., Jr., 1973, The to the Vardar root zone, and its presence 118. Vardar root zone [abs.]: EOS (Am. suggests that the root zone may be, in fact, Boccaletti, M., Manetti, P., and Peccerillo, A., Geophys. Union Trans.), v. 54, p. 461. "rootless," having been sheared off at 1974, The Balkanids as an instance of Temple, P. G., and Zimmerman, J., 1969, depth and transported southwestward from back-arc thrust belt: Possible relation with Tectonic significance of alpine ophiolites in its original location. the Hellenids: Geol. Soc. America Bull., Greece and Turkey: Geol. Soc. America v. 85, p. 1077-1084. Abs. with Programs, v. 1, p. 221. Dercourt, J., 1972, The Canadian Cordillera, the Zimmerman, J., Jr., 1972, Emplacement of the ACKNOWLEDGMENTS Hellenides, and the sea-floor spreading Vourinos ophiolitic complex, northern theory: Canadian Jour. Earth Sci., v. 9, Greece, in Shagam, R., and others, eds., We thank B. C. Burchfiel, N. L. Carter, p. 709-743. Studies in Earth and space sciences [Hess and D. H. Roeder for reviewing the manu- Godfriaux, I., 1962, L'Olympe: Une fenêtre volume]: Geol. Soc. America Mem. 132, p. 225-239. script. tectonique dans les Hellénides internes: This work was partly supported by Na- Acad. Sci. Comptes Rendus, v. 225, tional Science Foundation Grant GA-31569 p. 1761-1763. to N. L. Carter. Hynes, A. J., Nisbet, E. G., Smith, A. G., and Welland, M.J.P., 1972, Spreading and Discussions with B. C. Burchfiel, N. L. emplacement ages of some ophiolites in the Carter, R. F. George, E. Chiotis, G. Othris region (eastern central Greece): Papadopoulos, and C. E. Tsoutrelis and Deutsch. Geol. Gesell. Zeitschr., v. 123, logistical support by the Institute for Geol- p. 455. MANUSCRIPT RECEIVED BY THE SOCIETY SEP- ogy and Subsurface Research, Athens, Mercier, J., 1973, Contribution à l'étude du TEMBER 2, 1975 Greece, are gratefully acknowledged. métamorphisme et de l'évolution mag- MANUSCRIPT ACCEPTED JANUARY 27, 1976

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