Uwe Ring

An active bivergent rolling-hinge detachment system: Central Menderes metamorphic core complex in western Turkey

Two symmetrically arranged detachment systems delimit the central Menderes metamorphic core complex and define a bivergent continental breakaway zone in the Anatolide belt of western Turkey. Structural analysis and apatite fission-track thermochronology show that a large east-trending syncline within the Alpine nappe stack in the central part of the orogen is related to late Miocene‐early Pliocene to recent core-complex formation. The syncline formed as a result of two opposite-facing rolling hinges in the footwalls of each of the two detachments. Back-rotation of the syncline limbs suggests that the detachments rotated from an initial dip of 408‐608 to a currently shallow orientation of 08‐208.

Miocene NNE-directed extensional unroofing in the Menderes Massif, southwestern Turkey

Structural investigations in the central part of the Menderes Massif (Ödemiº-Kiraz submas-sif) reveal the presence of a large-scale, low-angle extensional shear zone with a top-to-the-N-NE shear sense. Regional ductile deformation was accompanied by the intrusion of two syntectonic granodiorites that have been dated with the 40Ar/39Ar method. One hornblende isochron age of 19.5 ± 1.4 Ma and two biotite plateau ages of 13.1 ± 0.2 and 12.2 ± 0.4 Ma, respectively, constrain that extension was already active in the early Miocene. Successive tectonic denudation of the Ödemiº-Kiraz submassif resulted in the formation of a N-dipping detachment fault, in which ductile fabrics were severely reworked by cataclasis under decreasing temperature. Syntectonic Neogene sediments, currently exposed along the southern margin of the Gediz Graben, were deposited in the hanging wall of the extensional fault system and were tectonically emplaced onto the cataclasites during progressive exhumation. Minor rotation (< 15°) of the detachment fault to its present gentle dip of c. 15° caused southward tilting of the sediments. Ongoing NNE-directed extension created a steep normal fault that truncates the detachment fault and constitutes the southern boundary fault of the Gediz Graben.

Bivergent extension in orogenic belts: The Menderes massif (southwestern Turkey)

The central Menderes massif is characterized by an overall dome-shaped foliation pattern and a north-northeast‐trending stretching lineation. The asymmetry of shear bands and quartz c-axis fabrics on either side of the structural dome demonstrate a top to thenorth-northeastshearsenseinthenorthernpartandatoptothesouth-southwestshear sense in the southern part of the submassif, i.e., a bivergent downdip movement. This suggests a symmetric collapse of the Alpine Menderes orogenic belt along two extensional shearzones.Conjugateshearbandsandsymmetricquartzc-axisfabricsintheeast-trendingtransitionzonedemonstrateacoaxialdeformationbetweenthetwoextensiondomains. Bivergent extension in the Menderes massif is in contrast to asymmetric extension in the AegeanSea.Here,thestill-activeHellenicsubductionzoneevolvedfromanadvancingplate boundary associated with crustal thickening into a retreating plate boundary in Oligocene-Miocene time. Southward rollback of the subducting plate during continuous northward subduction allowed asymmetric top to the north-northeast extension in the back-arc region during the exhumation of the Cycladic core complexes. In western Turkey, the arrival of the thick continental crust of the Menderes massif halted subduction and probably caused the symmetric collapse of the massif because the high potential energy of the thickened crust was no longer supported by subduction.

The Menderes Massif of western Turkey and the Cycladic Massif in the Aegean—do they really correlate?

Based on lithostratigraphic comparisons the Menderes Massif has been correlated with the Cycladic Massif, thereby implying that the eastern Mediterranean consists of a narrow pre-Alpine basement belt which is laterally continuous over a long distance and which experienced a similar Alpine orogenic history. Our work indicates that the architecture, the age of basement and the pre-Alpine and Alpine tectonometamorphic history of both massifs differ fundamentally from each other. The Menderes Massif consists of remnants of the Cycladic Massif which overly an exotic unit, the Menderes nappes. Both massifs do not represent lateral continuations which has implications for palaeogeographic reconstructions.

Structural analysis of a complex nappe sequence and late-orogenic basins from the Aegean Island of Samos, Greece

The island of Samos in the Aegean Sea exposes high-pressure metamorphic rocks of the Cycladic blueschist unit which are sandwiched between the mildly blueschist-facies Kerketas nappe below and the overlying non-metamorphic Kallithea nappe. Structural and metamorphic analysis shows that deformation can generally be divided into four main stages: (1) Eocene and earliest Oligocene 0ESE‐WNW-oriented nappe stacking (D1 and D2) associated with blueschist- and transitional blueschist‐ greenschist-facies metamorphism (M1 and M2). D2 caused emplacement of the blueschist unit onto the Kerketas nappe indicating that thrusting occurred during decompression. (2) A subsequent history of Oligocene and Miocene horizontal crustal extension (D3) before and after greenschist-facies metamorphism (M3). Ductile flow during D3 was characterized by a high degree of coaxial deformation but in general caused displacement of upper units towards the ENE. Nonetheless, the late-stage D3 emplacement of the Kallithea nappe between 9 and 10 Ma had a top-to-the-NW/NNW sense of shear. (3) A short period of brittle E‐W crustal contraction (D4) occurred between <8.6 and 09 Ma. (4) A phase of N‐S-directed normal faulting (D5, <8.6 Ma to Recent). ESE‐WNW-directed tectonic transport during D1 through D3 is in contrast to uniform NNE‐SSWdirected tectonic transport in the adjacent Cyclades, Greece, as well as in the neighbouring Menderes Massif of western Turkey. Published paleomagnetic data reveal sinistral rotation between the Cyclades and western Turkey. We interpret this rotation as a consequence of diAerential extension between the severely extended Aegean and the moderately extended Menderes Massif during D3. The onset of D3 crustal extension is coeval with a marked change in the thermal structure. We propose that the thermal reorganization was associated with the retreat of the subduction zone towards the external Hellenides in the Early Oligocene and a subsequent increase in magmatic activity. # 1999 Elsevier Science Ltd. All rights reserved.

Structural and thermal history of poly-orogenic basement: U–Pb geochronology of granitoid rocks in the southern Menderes Massif, Western Turkey

Ion microprobe U–Pb dating of granitoid rocks from key structural outcrops of the Menderes Massif in western Turkey provides an important constraint to the thermal and deformational history of a structurally complex metamorphic belt within the Alpine chain. Crystallization ages of two granite protoliths, derived from the weighted means of rim ages and the ages of homogeneous prismatic zircon grains, are 541 ± 14 Ma and 566 ± 9 Ma, whereas the cores of zoned pyramidal and short-prismatic zircon grains range from Palaeoproterozoic to Neoproterozoic in age. These ages indicate that amphibolite- to granulite-facies metamorphic rocks in much of the Menderes Massif were deformed, metamorphosed and intruded during the Pan-African Orogeny, and neither crystallized nor remelted during an Alpine event. Structural and metamorphic evidence for Alpine convergence in Pan-African basement rocks is limited to greenschist-facies top-to-the-south shear zones, which occur on a regional scale across a number of tectonic units.

Miocene high-pressure metamorphism in the Cyclades and Crete, Aegean Sea, Greece: Evidence for large-magnitude displacement on the Cretan detachment

The Cyclades in the backarc region of the present Hellenic subduction zone are known for widespread Late Cretaceous to Eocene high-pressure metamorphism in the Cycladic blueschist unit. We report 40 Ar/ 39 Ar and Rb/Sr phengite ages of 24–21 Ma for high- pressure metamorphism (8–10 kbar, 350–400 °C) in the lowest tectonic unit in the Cyclades, the Basal unit, which structurally underlies the Cycladic blueschist unit. The Basal unit is correlated with the Tripolitza unit of the External Hellenides in the forearc region of the Hellenic subduction zone. The Tripolitza unit is unmetamorphosed on Crete, where it is separated from the underlying high-pressure (8–10 kbar, 300–400 °C) Plattenkalk and Phyllite-Quartzite units by the extensional Cretan detachment. The age for high- pressure metamorphism in the latter units is similar to our age for the Basal unit in the Cyclades. Because pressure-temperature conditions in the Plattenkalk and Phyllite- Quartzite units on Crete and the Basal unit in the Cyclades are also similar, they must have been in close proximity in the early Miocene Hellenic subduction zone. A palinspastic reconstruction suggests a subsequent displacement of >100 km on the Cretan detachment. This is one of the greatest displacement magnitudes ever reported from detachment faults. Because of this large offset, the Cretan detachment was an efficient agent for exhuming high-pressure rocks.

The influence of preexisting structure on the evolution of the Cenozoic Malawi rift (East African rift system)

This paper analyzes the importance of preexisting structure for the evolution of the Cenozoic Malawi rift, which constitutes the southernmost part of the western branch of the East African rift system. Kinematic analyses demonstrate that the regional extension direction rotated clockwise from ENE to SE during rifting. Cenozoic rift faults (of dip-, oblique-, and strike-slip character) rejuvenated crustal structures whenever those were properly oriented with respect to the extension directions. Depending on these boundary conditions the rift attempts to find the easiest pathway in order to develop in a mechanically modest way. Proterozoic basement structures represent the basic anisotropy which already influenced pre-Cenozoic rifts and also the Cenozoic Malawi rift. Amongst these basement structures, Pan-African retrograde to cataclastic, strike-slip shear zones have the strongest influence on later rift structures. Rift-related transfer faults appear as differently striking passive structures, and their orientation seems to be controlled by a variety of steep basement structures (retrograde shear zones, foliation planes, and fracture arrays).

Normal vs. strike-slip faulting during rift development in East Africa: The Malawi rift

Kinematic analysis of Neogene and Quaternary faults demonstrates that the direction of extension in the Malawi rift rotated from east-northeast to southeast. Rift development commenced with the formation of half-grabens bounded by northwest-, north-, and northeast-striking normal faults. Owing to slightly oblique rifting, the northwest-striking faults in the northernmost rift segment show a small dextral oblique-slip component, whereas north- and northeast-oriented faults in the central part of the rift display a sinistral oblique-slip component. This first event resulted in block faulting and basin subsidence, which is largely responsible for the present-day basin morphology of Lake Malawi. A major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes. The structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation, and strike-slip faulting assumed a major role. The strike-slip regime amplified uplift of basement ridges within the rift in regions of local transpression, but it also created alluvial basins because of local transtension. This new kinematic style is compatible with the recent seismicity. Older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift. Toward the center, the faults depict an increase in strike-slip components of movement, suggesting deformation propagation toward the rift center, which results in a narrowing of the active rift environments with time.

U–Pb SIMS dating of synkinematic granites: timing of core-complex formation in the northern Anatolide belt of western Turkey

Secondary ion mass spectrometry (SIMS) U–Th–Pb dating of magmatic zircon from the synkinematic Eğrigöz and Koyunoba granites and a leucogranite dyke dates core-complex formation in the northern Anatolide belt of western Turkey at 24–19 Ma. The granites intrude into the footwall of the Simav detachment and are strongly elongated in the NNE direction parallel to tectonic transport on the detachment. Although large parts of the granites are undeformed, localized mylonitic to ultramylonitic deformation occurs directly beneath the Simav detachment and preserves evidence of progressive deformation from ductile to brittle conditions. Oscillatory zoned rims of long-prismatic zircon from the Eğrigöz and Koyunoba granites yield identical and well-constrained intrusion ages of 20.7 ± 0.6 Ma and 21.0 ± 0.2 Ma, whereas inherited grains range from Palaeoproterozoic (2972 ± 13 Ma) to Neoproterozoic (653 ± 6 Ma to 500 ± 5 Ma) in age. A leucogranite dyke yields an intrusion age of 24.4 ± 0.3 Ma, with inherited Neoproterozoic (640 ± 7 Ma to 511 ± 6 Ma) grains. Our data, in conjunction with published 40Ar/39Ar biotite ages, indicate very rapid cooling (greater than c. 200 °C Ma−1) for the granites during and after synkinematic emplacement.