RESEARCH

Miocene bivergent crustal extension in the Aegean: Evidence from the western ()

Bernhard Grasemann1*, David A. Schneider 2, Daniel F. Stöckli 3, and Christoph Iglseder 4 1DEPARTMENT OF GEODYNAMICS AND SEDIMENTOLOGY, UNIVERSITY OF VIENNA, A-1090 VIENNA, AUSTRIA 2DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF OTTAWA, OTTAWA, ONTARIO K1N 6N5, CANADA 3DEPARTMENT OF GEOLOGICAL SCIENCES, JACKSON SCHOOL OF GEOSCIENCES, UNIVERSITY OF TEXAS, AUSTIN, TEXAS 78712, USA 4GEOLOGICAL SURVEY OF AUSTRIA, A-1030 VIENNA, AUSTRIA

ABSTRACT

Current models for Miocene backarc extension of the Aegean region generally suggest that stretching was accommodated mainly by NE- dipping low-angle normal faults with N to NE sense of shear. A crustal-scale low-angle normal fault system trending over a length of more than 200 km forms the North Cycladic detachment system, which records a NE-directed normal shear sense separating the Cycladic Blue- schist unit in the footwall from the Upper Cycladic unit in the hanging wall. Based on new structural fi eld data, we propose the existence of another large-scale low-angle normal fault system, the West Cycladic detachment system, which is exposed on , , and , strikes over a length of at least 100 km, and has a possible extension to the SE, where the existence of a South Cycladic detachment system has been recently postulated. The West Cycladic detachment system shares many similarities with the North Cycladic detachment system, with the notable exception that the structure dips toward the SW with top-to-the-SSW kinematics. New 40Ar/39Ar and U-Th/He thermochro- nological data suggest that the West Cycladic detachment system accommodated extension throughout the Miocene. Since both the North and the West Cycladic detachment systems were active until the late Miocene but exhibit opposing shear sense, we propose that a large part of the stretching of the Aegean crust was accommodated by these two bivergent crustal-scale detachment systems.

LITHOSPHERE; v. 4; no. 1; p. 23–39; Data Repository Item 2012031. doi: 10.1130/L164.1

INTRODUCTION complexes (Lister et al., 1984), which were et al., 2009; Tschegg and Grasemann, 2009; exhumed along low-angle normal faults. The Brichau et al., 2010). Complementary studies The pattern of lithospheric extension, whether timing, orientation, and kinematics of the low- on other western Cycladic islands such as Kea it is symmetric or asymmetric, is a key factor in angle normal faults, however, have been a mat- to the north (Iglseder et al., 2011) have also doc- understanding of the dynamics of rifts, passive ter of debate (cf. Jolivet and Brun, 2010; Ring umented pervasive S-directed kinematics across margins, backarc extension, and metamorphic et al., 2010), with a general consensus that the the island, but the regional correlation of these core complex formation. Numerous kinematic Miocene low-angle normal faults dip toward the fault systems, their lateral extent, and the onset models suggest either symmetric extension N-NE, recording a top-to-the-N or -NE sense of extension are unknown. In this contribution, under pure shear far-fi eld strain (Mc Kenzie, of shear (e.g., Avigad and Garfunkel, 1989; we present, for the fi rst time, evidence for a top- 1978) or asymmetric simple shear (Wernicke, Buick, 1991; Faure et al., 1991; Lee and Lister, to-the-SW detachment on the island of Kythnos, 1985; Govers and Wortel, 1993), or combina- 1992; Gautier et al., 1993; Gautier and Brun, which is located between Kea and Serifos. We tions of both models (Lister et al., 1986). More 1994; Jolivet and Patriat, 1999; Vander haeghe, also complement the existing database in the complex dynamic models are able to predict 2004; Mehl et al., 2007). Recently, it has also western Cyclades with new 40Ar/39Ar mica and both symmetric and asymmetric extension, been suggested that the low-angle normal faults (U-Th)/He zircon and apatite thermochrono- emphasizing the importance of the initial geom- in the northern Cycladic islands link up to logical data combined with structural fi eld data etry, preexisting anisotropies, thermal histories, form the crustal-scale North Cycladic detach- from the islands of Serifos, Kythnos, and Kea. rheology, and shear localization (e.g., Bertotti et ment system, with a strike length of more than The objective here is to expand upon the detailed al., 2000; Huismans and Beaumont, 2002; Gess- 200 km (Jolivet et al., 2010). Other models favor island-specifi c investigations and integrate those ner et al., 2007; Regenauer-Lieb et al., 2008). a more complex evolution, suggesting that an data and observations with the recent and suc- In order to compare such models with natural earlier Oligocene S-directed sense of shear has cinct summaries of Jolivet and Brun (2010) and examples of crustal extension, researchers have been overprinted by Miocene N-directed shear Ring et al. (2010) and with recent observations been investigating the Cyclades of the Aegean sense (Forster and Lister, 2009), or Oligocene of major detachment systems (Jolivet et al., region for the past three decades (e.g., Lister et N-directed shear, which has been overprinted 2010; Ring et al., 2011). Our new data allow us al., 1984; Buick, 1991; Gautier and Brun, 1994; by Miocene S-directed shear (Ring et al., 2011). to suggest that the S-directed kinematics of the Tirel et al., 2009; Huet et al., 2011), where Mio- Structural data from Serifos, located in the west- low-angle normal faults identifi ed on the islands cene backarc extension has led to the formation ern Cyclades, document the S-directed kinemat- are mechanically linked and form the West of several Cordilleran-type metamorphic core ics of a low-angle normal fault, which partly Cycladic detachment system, which, together cuts a ca. 10 Ma late-tectonic granodiorite intru- with the North Cycladic detachment system, *E-mail: [email protected]. sion (Grasemann and Petrakakis, 2007; Iglseder accommodated Miocene bivergent extension in

LITHOSPHEREFor permission to| Volumecopy, contact 4 | Number [email protected] 1 | www.gsapubs.org | © 2012 Geological Society of America 23

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

the Aegean. The existence of a South Cycladic retreating African slab (Le Pichon et al., 1981) indicate a shortening component perpendicu- detachment system (Ring et al., 2011) is criti- generated middle to late Miocene, predominantly lar to the extension direction also described on cally discussed as a possible extension of the I-type plutonism on several of the islands where other Cycladic islands (Avigad et al., 2001). West Cycladic detachment system. fi nal intrusion stages are syn- to postkinematic At structurally higher levels, the SCC′ fab- with respect to the pervasive deformation fabric ric in phyllonites becomes more intense and GEOLOGICAL SETTING (Altherr et al., 1982; Keay et al., 2001; Pe-Piper is gradually overprinted by cataclastic defor- and Piper, 2002; Iglseder et al., 2009). Although mation. Above the cataclasite zone, an up to The Cyclades are part of the Hellenides, most of the hanging wall of the low-angle normal 50-m-thick ultramylonitic marble discordantly which are made up of a tectonic pile of several faults has been eroded, isolated klippen are pre- overlies the greenschists, appearing as isolated thrust units, consisting of, from top to base (and served on several Cycladic islands. These rocks, klippen, which can be traced over the island north to south), the Pelagonian, Pindos, Gavrovo- which are part of the Pelagonian unit, consist (Fig. 2C). Observations of sheath folds and per- Tripolitza, Phyllite-Quartzite, and Ionian thrust of a Paleozoic basement with a Paleozoic and vasively ultrafi ne-grained (<10 μm) fabric sug- units (Jacobshagen, 1986; Bonneau, 1984; van Mesozoic carbonate cover overlain by a Jurassic gest that the marble ultramylonites are in fact Hinsbergen et al., 2005). The dominant sequence ophiolite (Bonneau, 1982), and they have locally a ductile high-strain zone. Our interpretation is the Cycladic Blueschist unit, which is consid- endured a Late Cretaceous high-temperature is supported by abundant fold axes of isoclinal ered to be a paleogeographic equivalent of the metamorphic event (e.g., Reinecke et al., 1982; folds that are parallel to the mylonitic stretch-

Pindos unit (Bonneau, 1982), and which consists Altherr et al., 1994; Zeffren et al., 2005). Next, ing lineation (lm2), with the axial planes paral-

of a polymetamorphic Carboniferous–Permian to we briefl y describe the geology, new structural lel to the mylonitic foliation (sm1+2). Such folds latest Cretaceous passive-margin sequence that fi eld observations, and new thermochronological have been shown to form during shearing and tectonically overlies the Cycladic Basement unit, data from the islands of Kea, Kythnos, and Seri- layer-parallel thinning of the whole sequence, a Paleozoic basement diversely metamorphosed fos in the western Cyclades (Fig. 1). indicating high-strain shear deformation (e.g., and intruded by Triassic granitoids (e.g., Dürr et Morales et al., 2011). Although the ductile strain al., 1978; Blake et al., 1981). In the Lavrion area Kea is localized in the marbles under greenschist- and in some tectonic windows of the Cyclades, facies conditions, the marble ultramylonites the Basal unit, which represents a remnant of a The rocks on Kea, the northwesternmost of likely behaved as an extremely rigid slab dur- Late Triassic to Late Cretaceous carbonate plat- the Cycladic islands, consist mainly of green- ing exhumation through the upper crust, where form (Avigad and Garfunkel, 1989), is exposed schists, quartz-rich mica schists, and horizons cataclastic deformation is localized on both the below the Cycladic Blueschist unit. The Basal of marbles. Based on the rare preservation of upper and lower contacts (Fig. 2C). Polyphase unit shows evidence of high-pressure–low-tem- glaucophane, Kea has been suggested to be cataclasites are commonly overprinted by disso- perature metamorphism (Shaked et al., 2000). part of the Cycladic Blueschist unit (Davis, lution-precipitation creep and vice versa. Ultra- Early Miocene isotopic ages have been inter- 1982; van der Maar and Jansen, 1983), and in cataclasites with rounded and partly polished preted as indicating the timing of the high-pres- scarce, isolated blueschist lenses, a relic min- clasts are separated along detachment-parallel

sure metamorphism or of the greenschist-facies eral lineation (lm1) trending roughly W-E is slickensides from protocataclasites, which are overprint (Bröcker and Franz, 1998; Ring et al., preserved (Iglseder et al., 2011). The island both strongly overprinted by stress-induced dis- 2001; Ring and Reischmann, 2002; Bröcker et was strongly overprinted by SW-directed shear- solution and mass transfer processes (Fig. 2D). al., 2004). The Basal unit is considered to be part ing under greenschist-facies conditions, form- Continuous, anastomosing foliation formed by

of the External Hellenides (Avigad et al., 1997). ing a strong stretching lineation (lm2) trending pressure solution in the velocity-strengthening

The rocks of the Cycladic Blueschist unit NE-SW (Fig. 1A). Pervasive foliation (sm1+2) regime is overprinted by typical microstructures were exhumed in two successive stages corre- across the island defi nes a dome-like structure in the velocity-weakening regime, such as cha- sponding to two metamorphic episodes (Jolivet with several meter-long subhorizontal shear otic, unfoliated cataclasite with a large variation and Brun, 2010, and references therein), with zones, with C′-type and shear band fabrics in grain size (Niemeijer and Spiers, 2007). Sepa- the fi rst stage occurring during Eocene Hel- indicating top-to-the-SW shearing. High-angle, rated by a zone of intense ultracataclastic defor- lenic subduction, exhuming blueschist- and SW-dipping ductile-brittle faults display a mation, ankeritized dolostones and limestones eclogite-facies assemblages (e.g., Altherr et al., strong displacement gradient, resulting in duc- cap some of the klippen representing the nearly 1979; Wijbrans and McDougall, 1988; Wijbrans tile drag of the main foliation. The high-angle unmetamorphosed protocataclastic remnants of et al., 1990; Bröcker et al., 1993; Trotet et al., faults mechanically interact with subhorizontal the Upper Cycladic unit (Iglseder et al., 2011). 2001; Ring and Layer, 2003), likely in an extru- shear zones indicating SW-directed shear sense We carried out a suite of 40Ar/39Ar and (U-Th)/ sion wedge (Ring et al., 2007, 2010; Huet et al., (Fig. 2A). Discrete horizons of marbles reveal He thermochronologic investigations to comple- 2011). During subsequent tectonism in the Oli- that the whole sequence is intensely folded and ment the age data for Kea reported in Iglseder gocene and Miocene, exhumation occurred as sheared, recording sheath folds at smaller scales et al. (2011) and to further understand the exhu- Cordilleran-type metamorphic core complexes (Fig. 2B) and forming type 2 and 3 refolded mation history of the western Cyclades. White (Lister et al., 1984) with mainly NE-dipping structures at larger scales. Open upright folds micas from the basement schists on Kea yield fl at 40 39 low-angle normal faults (i.e., “detachments”) with fold axes parallel to lm2 but with subver- Ar/ Ar age spectra with well-defi ned plateaus showing a dominant top-to-the-N or -NE sense tical NE-SW–trending axial planes overprint and signifi cant 39Ar gas release (>50%) that gave

of shear (e.g., Buick, 1991; Gautier et al., 1993; sm1+2 and are responsible for the dome shape of ca. 20–19 Ma ages. In the initial and fi nal heat- Vanderhaeghe, 2004). Latest extension in what the island. The fold limbs are partly overprinted ing increments, apparent ages increase slightly, is now the backarc region developing above the by the SCC′ fabric; therefore, the upright folds but otherwise the spectra remain fl at (Table A11;

1GSA Data Repository Item 2012031, 40Ar/39Ar white mica analytical data from the western Cyclades and Ar-Ar methodology and (U-Th-Sm)/He methodology, is available at www.geosociety.org/pubs/ft2012.htm, or on request from [email protected], Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

24 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

265000 E 270000 E 270000 E Otzias N N KEA 50000 N KYTHNOS

18.5±0.5 41 5 km 5 km Korissia Loutra a az az z z 4170000 N a a Kythnos z 18.8±0.1 Merichas a z 21.2±0.1 19.9±0.7 sm1+2 (220) sm1+2 (181) lm (167) Pisses 2 lm1 (129)

17.0±0.4 00 N lm2 (63)

41400 a az 18.9±0.1 20.7±0.1 az 18.9±2.4

4160000 N Ag. Dimitrios az a 19.9±0.1 A 19.5±0.1 B a

275000 E PlatyYialos Quaternary sediments Shear sense N LANFLANF lm1 Granodiorite (ca. 11–9 Ma) lm2

Upper Cycladic unit a: apatite Brittle/ductile fault rocks z: zircon 8.9±0.1 including hanging wall 4116000 N 4116000 LANFLANF 11.2±0.5 8.5±0.1 Cycladic Blueschist unit 40 39 sm1+2 (139) Ar/ Ar Livadhi Schists, marbles, amphibolite 8.4±0.1 lm1 (101) (U-Th)/He>15 Ma lm (66) 8.9±0.1 2 LANFLANF (U-Th)/He 10-15 Ma 12.9±0.4 Basal unit (U-Th)/He<10 Ma Kavos Kiklopas 9.3±0.2 SERIFOS Mylonitic gneisses, marbles C 5 km

Figure 1. Simplifi ed geological maps of the western Cycladic islands of (A) Kea, (B) Kythnos, and (C) Serifos. Stereonets (lower hemisphere)

give the orientations of the main foliations (sm1+2—gray diamonds) and an earlier stretching lineation (lm1—white circles) overprinted by a

later stretching lineation (lm2—red circles). Lineations are also plotted on the maps as arrows (lm1—white; lm2—red) exhibiting the shear sense (displacement of the structural higher levels). Locations for thermochronology samples are also indicated on the map. The 40Ar/39Ar white mica age is indicated by circles. Zircon (z) and apatite (a) (U-Th)/He age groups are indicated with an open square (>15 Ma), half solid square (10–15 Ma), and a solid square (<10 Ma). All (U-Th)/He ages shown on the Serifos map are results from apatite analyses; zircon ages are reported in Table 2. All 40Ar/39Ar white mica ages from northern and western Serifos are 38–35 Ma. See Iglseder et al. (2011) and Schneider et al. (2011) for additional thermochronology. LANF—low-angle normal fault. Coordinates are given in WGS84/UTM zone 35N.

GSA Data Repository has complete methodol- and apatite to understand the lower temperature- anomalously young ages (younger than AHe ogy [see footnote 1]). K/Ca ratios follow a pattern time history of the western Cyclades. Single-grain ages), likely due to high-U overgrowths, a min- that shows a rise in the initial segments, which triplicate analyses of the accessory minerals were eralogical phenomenon that is not uncommon mimics the apparent age trend, relatively constant conducted, and results are presented in Tables 1 within the Cycladic domain (Keay et al., 2001; values across the plateau segments, and a change and 2 (GSA Data Repository has complete meth- Schneider et al., 2011). in the fi nal steps to a decrease in K/Ca ratio odology [see footnote 1]). Apatite (U-Th)/He antithetical to the apparent age trend. Notably, ages from the Cycladic Blueschist unit basement Kythnos somewhat younger samples lie close to (ultra) range between 8 and 5 Ma, while apatite from mylonitic marbles of low-angle normal fault zone two samples of the structurally uppermost levels Kythnos Island is predominantly composed or detachment phyllonites, which recrystallized yield slightly older ages (14 Ma), recording mid- of greenschist sequences intercalated with the fabric-forming micas (Iglseder et al., 2011). Miocene exhumation. Surprisingly, zircons from decimeter-thick gray marbles, which belong to We also performed (U-Th)/He analyses on zircon the Cycladic Blueschist unit samples often yield the Cycladic Blueschist unit (De Smeth, 1975).

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 25

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

A B

SSWW NENE WEW E C D

Dol/CcDol/Cc ((proto)cataclasitesproto)cataclasites KythnosKythnos C PPCC 300 m mmarblearbb ultramylonitesra yloy ites UUCC 1 m phyllonites/phyllonites/ ultracataclasitesultracataclasites

FootwallFootwall ggneissesneisses andand ggreenschistsreenschists N S NWNW SES E F

LLocalizedoca zed sshearhear zzonesones

c′

NENE SSWW NEN SWW

Figure 2. (A) High-angle ductile-brittle faults (221°/45°) with ductile drag of the main foliation (045°/31°, N Kea, 4171212N, 271227E). (B) Sheath folds in highly strained marble mylonites (W Kea, 4165058N, 259529E). (C) Marble ultramylonite “klippe” at the S head- land of Kea (compare Fig. 1A; 4156347N, 259616E). Cataclastic deformation is localized above and below the marble ultramylonites. Dol/Cc—dolomite-calcite. (D) Several generations of cataclasites in the marble mylonites along the detachment on southern Kea (4158759N, 261207E, looking toward NNE parallel to the movement direction). Gray arrows indicate rounded and partly polished clasts. Stylolites are outlined by white arrows and solid lines. UC—ultracataclasite; PC—protocataclasite; C—cataclasite. (E) σ-type “clasts” in chlorite/epidote gneiss with distinct stair-stepping (main foliation: 082°/14°, stretching lineation: 061°/10°). The “clast” developed from a feldspar/chlorite vein, which has been strongly rotated and extended into pinch-and-swell boudins indicating SW- directed shear (W Kythnos, 4138451N, 270219E). (F) Pinch-and-swell shear-band (C′) boudinage of a quartz vein in coarse-grained (0.5 mm) blue marbles (main foliation: 108°/21°, stretching lineation: 068°/10°). White arrows indicate localization of fi ne-grained white shear zones. Note the pervasive brittle fracturing of the quartz layer (E Kythnos, 4141812N, 276495E).

26 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

TABLE 1. (U-Th-Sm)/He ANALYTICAL DATA FROM APATITE, WESTERN CYCLADES Sample Age ± (Ma) U Th Sm Th/U He Mass Ft Std dev (Ma) (ppm) (ppm) (ppm) (nmol/g) (µg)

Kea KEA0701, 4160758°N, 262048°E, schist KEA0701-1 7.9 0.5 0.1 2.5 1.9 40.8 0.02 6.9 0.60 KEA0701-2 6.9 0.4 0.2 1.9 2.5 10.0 0.02 7.4 0.61 KEA0701-3 7.7 0.5 0.2 2.4 2.0 13.2 0.02 7.0 0.60 average 7.5 0.5 0.1 2.3 2.1 21.3 0.02 7.1 0.60 0.5 KEA0703, 4165418°N, 265949°E, schist KEA0703-1 14.6 0.9 0.2 2.2 8.0 9.2 0.04 13.5 0.67 KEA0703-2 14.8 0.9 0.4 2.8 17.8 6.7 0.06 12.0 0.66 KEA0703-3 12.8 0.8 0.6 2.0 16.6 3.2 0.06 17.3 0.70 average 14.0 0.8 0.4 2.4 14.1 6.4 0.06 14.3 0.68 1.1 KEA0705, 4167887°N, 264422°E, schist KEA0705-1 13.7 1.9 0.1 1.0 4.0 13.9 0.02 21.5 0.70 KEA0705-2 13.8 2.3 0.0 1.1 5.4 22.6 0.02 20.6 0.70 KEA0705-3 14.0 1.9 0.1 1.1 4.3 18.6 0.02 21.9 0.70 average 13.8 2.0 0.1 1.1 4.6 18.4 0.02 21.3 0.70 0.2 KEA07_10h, 4169315°N, 260942°E, schist KEA07_10h-1 9.4 0.6 0.2 1.9 15.2 9.2 0.03 8.0 0.64 KEA07_10h-2 6.8 0.4 0.6 5.0 24.2 8.3 0.05 6.4 0.62 KEA07_10h-3 6.0 0.4 0.2 1.8 12.9 8.9 0.02 8.6 0.65 average 7.4 0.4 0.3 2.9 17.4 8.8 0.03 7.7 0.64 1.8 Kythnos 07KO08, 4147573°N, 272067°E, schist 07KO08-1 11.9 0.7 0.2 1.6 8.8 7.5 0.03 17.6 0.68 07KO08-2 8.5 0.5 0.3 1.9 11.0 6.1 0.03 16.7 0.68 07KO08-3 13.9 0.8 0.5 1.8 25.5 3.9 0.05 14.2 0.66 average 11.4 0.7 0.3 1.8 15.1 5.8 0.04 16.2 0.67 2.7 07KO18, 4144358°N, 275719°E, schist 07KO18-1 14.8 0.9 0.2 2.8 21.8 11.9 0.06 6.9 0.66 07KO18-2 12.6 0.8 0.2 5.4 13.5 32.5 0.07 6.4 0.65 07KO18-3 12.4 0.7 0.3 0.9 9.8 3.6 0.03 8.3 0.67 average 13.3 0.8 0.2 3.0 15.1 16.0 0.05 7.2 0.66 1.3 07KO21, 4138543°N, 0271921°E, schist 07KO21-1 12.2 0.7 0.4 3.4 48.8 9.1 0.06 8.9 0.63 07KO21-2 13.1 0.8 0.3 4.4 34.9 12.9 0.07 9.2 0.63 07KO21-3 10.5 0.6 0.2 2.1 16.2 11.9 0.03 12.1 0.65 average 11.9 0.7 0.3 3.3 33.3 11.3 0.06 10.1 0.63 1.3 07KO22, 4137436°N, 268269°E, gneiss 07KO22-1* 51.9 3.1 0.1 –0.4 0.5 –3.2 0.01 2.6 0.57 07KO22-2* 265.5 15.9 0.2 –0.6 0.6 –3.0 0.05 2.1 0.54 07KO22-3 10.1 0.6 270.0 65.9 3.1 0.2 9.18 2.7 0.59 average 10.1 0.6 270.0 65.9 3.1 0.2 9.18 2.7 0.59 – 07KO25, 4133406°N, 267837°E, schist 07KO25-1 17.5 2.9 0.2 2.5 17.1 16.2 0.06 16.3 0.67 07KO25-2 27.4 3.6 0.1 3.4 15.8 28.2 0.10 17.0 0.67 average 17.5 2.9 0.2 2.5 17.1 16.2 0.06 16.3 0.67 – 07KO26, 4146620°N, 270836°E, quartz schist 07KO26-1 12.7 0.8 0.1 0.8 1.3 6.9 0.01 8.6 0.63 07KO26-2 13.3 0.8 0.3 2.9 10.0 9.2 0.05 11.0 0.65 07KO26-3 13.7 0.8 0.2 3.2 5.0 19.2 0.05 11.2 0.65 average 13.2 0.8 0.2 2.3 5.4 11.8 0.04 10.3 0.64 0.5 07KO29, 4132150°N, 266422°E, quartz schist 07KO29-1 15.8 0.9 0.0 1.9 4.0 275.5 0.03 8.7 0.60 07KO29-2 19.1 1.1 0.0 1.9 3.7 145.1 0.03 6.9 0.58 07KO29-3 20.0 1.2 0.0 1.8 2.9 –268.6 0.03 7.6 0.59 average 18.3 1.1 0.0 1.9 3.5 50.7 0.03 7.7 0.59 2.2 07KO30, 4132476°N, 266924°E, quartz mica schist 07KO30-1 17.7 1.1 0.2 1.9 14.4 10.3 0.04 10.1 0.62 07KO30-2 16.6 1.0 0.1 2.8 13.4 27.5 0.05 10.1 0.62 07KO30-3 16.6 1.0 0.1 3.5 16.5 32.8 0.06 8.5 0.60 average 17.0 1.0 0.1 2.7 14.8 23.5 0.05 9.6 0.61 0.6 (continued )

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 27

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

TABLE 1. (U-Th-Sm)/He ANALYTICAL DATA FROM APATITE, WESTERN CYCLADES (continued ) Sample Age ± (Ma) U Th Sm Th/U He Mass Ft Std dev (Ma) (ppm) (ppm) (ppm) (nmol/g) (µg)

Serifos SER 504, 4117281°N, 278530°E, granodiorite SER 504-1 7.4 0.4 6.2 11.6 44.6 1.9 0.21 2.5 0.55 SER 504-2 8.1 0.5 5.9 13.1 31.3 2.2 0.26 5.2 0.63 SER 504-3 6.8 0.4 8.9 15.6 38.4 1.7 0.31 7.0 0.66 SER 504-4 8.4 0.5 6.2 11.7 27.2 1.9 0.24 3.1 0.58 average 7.7 0.5 6.8 13.0 35.4 1.9 0.25 4.4 0.61 0.7 SER 513, 4114781°N, 278988°E, granodiorite SER 513-1 7.5 0.5 12.8 36.5 76.4 2.9 0.65 13.4 0.72 SER 513-3 6.8 0.4 12.3 27.8 82.7 2.3 0.47 6.4 0.65 SER 513-4 9.4 0.6 11.4 23.9 73.5 2.1 0.56 4.3 0.62 average 7.9 0.5 12.2 29.4 77.53 2.40 0.56 8.0 0.66 1.4 SER 644, 4112919°N, 272377°E, schist SER 644-1* 2406.6 144.4 3.3 3.6 7.0 1.1 39.56 3.5 0.60 SER 644-2 10.1 0.6 1.1 0.4 2.0 0.4 0.04 6.6 0.68 SER 644-3 14.8 0.9 1.2 1.3 5.8 1.1 0.07 2.9 0.58 SER 644-4* 73.6 4.4 0.2 0.4 3.3 1.7 0.09 5.5 0.65 average 12.4 0.7 1.1 0.9 3.9 0.8 0.06 4.8 0.63 3.3 SER 70A, 4116391°N, 277541°E, granodiorite SER 70A-1 6.0 0.4 13.4 32.6 79.1 2.4 0.44 3.3 0.62 SER 70A-2 7.0 0.4 14.0 33.8 75.5 2.4 0.49 2.0 0.58 SER 70A-3 7.1 0.4 13.8 36.7 73.0 2.7 0.52 2.5 0.58 average 6.7 0.4 13.8 34.4 75.87 2.50 0.48 2.6 0.59 0.6 SER 501, 4116186°N, 279284°E, granodiorite SER 501-1 8.9 0.5 14.9 37.3 79.4 2.5 0.69 2.3 0.59 SER 501-2* 11.4 0.7 8.9 24.5 52.8 2.8 0.63 5.4 0.68 SER 501-3* 26.1 1.6 8.2 26.3 68.4 3.2 1.33 3.6 0.63 average 8.9 0.5 14.9 37.3 79.4 2.5 0.69 2.3 0.59 9.3 SER 514, 4114781°N, 278988°E, granodiorite SER 514-1 7.3 0.4 11.9 29.3 68.5 2.5 0.47 2.8 0.62 SER 514-2 5.9 0.4 10.2 24.3 64.3 2.4 0.31 2.0 0.58 SER 514-3 9.4 0.6 7.9 16.5 33.5 2.1 0.34 2.2 0.56 average 7.5 0.5 10.0 23.4 55.46 2.31 0.37 2.3 0.59 1.7 SER 116, 4112163°N, 276148°E, foliated granodiorite SER 116-1 3.7 0.2 11.1 25.2 67.4 2.3 0.19 1.5 0.54 SER 116-2 4.8 0.3 15.0 32.1 78.8 2.1 0.32 1.5 0.54 SER 116-3* 13.8 0.8 20.6 31.5 66.9 1.5 1.21 2.0 0.57 average 4.3 0.3 13.0 28.7 73.1 2.2 0.26 1.5 0.54 0.8 SER 119, 4112278°N, 274675°E, weakly foliated granodiorite SER 119-1 5.7 0.3 19.4 37.6 32.7 1.9 0.59 7.8 0.67 SER 119-2 6.1 0.4 12.2 25.9 42.9 2.1 0.46 18.4 0.75 SER 119-3 6.4 0.4 13.7 34.5 50.7 2.5 0.53 9.0 0.69 SER 119-4 6.7 0.4 8.9 24.0 42.5 2.7 0.40 16.4 0.74 average 6.2 0.4 13.6 30.5 42.2 2.3 0.49 12.9 0.71 0.4 SER 314, 4119757°N, 279998°E, schist SER 314-1* 10.7 0.6 0.3 0.3 2.5 0.8 0.02 8.6 0.70 SER 314-3 7.0 0.4 0.7 0.4 2.6 0.5 0.02 6.4 0.68 SER 314-4* 13.8 0.8 0.4 0.5 3.1 1.1 0.03 6.9 0.68 average 7.0 0.4 0.7 0.4 2.6 0.5 0.02 6.4 0.68 – SER 330, 4112409°N, 270672°E, schist SER 330-2 4.6 0.3 0.3 0.2 0.6 0.7 0.01 4.0 0.62 SER 330-3* -0.5 0.0 0.3 0.2 1.5 0.6 0.00 2.4 0.57 average 4.6 0.3 0.3 0.2 0.6 0.7 0.01 4.0 0.62 – SER 338, 4118451°N, 279527°E, schist SER 338-1* 40.6 2.4 0.2 0.8 3.1 3.3 0.07 7.4 0.67 SER 338-2 7.7 0.5 0.8 1.3 4.6 1.7 0.03 3.1 0.59 SER 338-3* 16.0 1.0 0.4 1.3 3.4 3.5 0.03 3.5 0.58 average 7.7 0.5 0.8 1.3 4.6 1.7 0.03 3.1 0.59 – SER 340, 4119075°N, 278348°E, schist SER 340-1* 14.6 0.9 1.1 1.0 7.3 1.0 0.06 2.6 0.57 SER 340-2 9.0 0.5 0.2 0.4 3.1 1.9 0.01 4.2 0.62 SER 340-3* 25.4 1.5 0.2 0.9 3.0 5.5 0.03 4.4 0.62 SER 340-4 4.4 0.3 0.3 1.6 2.8 5.8 0.01 5.9 0.65 average 6.7 0.4 0.2 1.0 3.0 3.8 0.01 5.0 0.63 3.3 (continued )

28 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

TABLE 1. (U-Th-Sm)/He ANALYTICAL DATA FROM APATITE, WESTERN CYCLADES (continued ) Sample Age ± (Ma) U Th Sm Th/U He Mass Ft Std dev (Ma) (ppm) (ppm) (ppm) (nmol/g) (µg) SER 347, 4117913°N, 278518°E, schist SER 347-1* 14.4 0.9 1.2 2.6 6.6 2.2 0.08 2.0 0.53 SER 347-2 8.3 0.5 1.8 4.2 9.6 2.3 0.07 2.5 0.56 SER 347-3 6.5 0.4 2.3 6.1 9.0 2.7 0.08 3.3 0.59 SER 347-4 24.3 1.5 1.8 1.8 7.6 1.0 0.18 3.0 0.59 average 7.4 0.4 2.1 5.1 9.3 2.5 0.08 2.9 0.57 1.3 SER 353, 4119427°N, 276436°E, schist SER 353-1 7.3 0.4 0.1 0.2 0.4 2.9 0.00 8.1 0.67 SER 353-2* 122.4 7.3 0.3 7.4 1.0 27.7 0.86 6.1 0.64 SER 353-4 3.2 0.2 0.4 0.3 0.8 0.8 0.00 4.6 0.64 average 5.2 0.3 0.2 0.2 0.6 1.8 0.00 6.4 0.66 2.9 SER 445, 4119855°N, 276092°E, schist SER 445-2 1.8 0.1 0.8 0.9 3.7 1.2 0.01 2.6 0.57 SER 445-3 1.2 0.1 0.9 4.1 9.8 4.8 0.01 2.2 0.54 SER 445-4 8.4 0.5 0.9 0.8 3.2 0.9 0.03 2.6 0.57 average 3.8 0.2 0.8 2.0 5.5 2.3 0.01 2.5 0.56 4.0 07SE04, 4120416°N, 279419°E, quartz mica gneiss 07SE04-1 11.0 0.7 0.6 2.3 7.1 3.8 0.05 18.1 0.70 07SE04-2 10.1 0.6 0.5 1.9 7.5 3.7 0.04 15.1 0.69 07SE04-3 12.2 0.7 0.4 1.9 11.8 4.4 0.04 17.4 0.69 average 11.1 0.7 0.5 2.0 11.8 3.98 0.04 16.9 0.70 1.1 07SE07, 4120605°N, 279251°E, quartz schist 07SE07-1* 14.0 1.1 0.1 –0.1 4.7 –2.1 0.00 4.4 0.49 07SE07-2* 39.5 3.2 0.0 0.2 3.6 –8.0 0.01 4.6 0.54 07SE07-3 7.8 0.6 1.7 4.2 7.6 2.5 0.06 4.4 0.52 average 7.8 0.6 1.7 4.2 7.6 2.5 0.06 4.4 0.52 – 07SE10, 4111489°N, 270639°E, mica gneiss 07SE10-1 10.7 0.9 0.2 0.9 5.9 3.7 0.02 10.0 0.63 07SE10-2 19.7 1.6 0.2 0.6 6.3 3.5 0.02 7.5 0.60 07SE10-3 15.3 1.2 0.3 1.0 6.2 2.9 0.03 12.0 0.65 average 15.2 1.2 0.2 0.8 6.1 3.4 0.02 9.8 0.63 4.5 Note: Aliquot analyses denoted by * excluded for weighted mean age due to likely zircon or monazite inclusion (high He yield during second He extraction).

Metabasic rocks occur as isolated lenses within trends NNE-SSW to NE-SW, parallel to isoclinal quartz at low-grade (i.e., <300 °C) metamorphic the greenschists, similar to Kea, and are com- intrafolial fold axes. The ultramylonitic marble is conditions (Fig. 2F). The contact of the marble posed of hornblende, magnetite, orthopyroxene, spatially associated with cataclasite layers, rang- mylonites to the overlying cataclasites is repre- and minor relics of sodium amphibole (Schliest- ing from proto- to ultracataclasites, localizing sented by slickensides, with slickenlines indicat- edt et al., 1994). Relics of high-pressure meta- along a knife-sharp low-angle plane above the ing S-directed shear. Dolomitic layers mainly morphism are manifested by glaucophane and marble mylonites. The cataclasite layers show deform by dilational fracturing and pervasive pseudomorphs of jadeite and garnet (Schliestedt a variation of compositions from marble ultra- precipitation of calcite veins. Calcite deforms et al., 1994). Deformation under these condi- cataclasites to protocataclastic quartzites, likely by dissolution-precipitation creep forming SCC′ tions is dominated by complex refolded struc- derived from a hanging wall that is dominated by structures. The C-planes are parallel to the main tures with dominantly NW-SE–trending fold strongly hydrothermally altered quartzites with detachment surface (Fig. 3A). In general, the axes and subhorizontal axial planes (Keiter et abundant iron oxide and barite mineralization. cataclasites are associated with massive vein

al., 2008). The stretching lineation (lm1) gener- Hence, both the underlying marble and the over- formation of different generations fi lled with ally trends WSW-ENE with a top-to-the-WSW lying quartzite act as source rock for the cata- calcite, quartz, or iron minerals. shear sense (Fig. 1B). clasite sequence. Brittle and ductile fault rocks Stretching parallel and shortening perpen-

The dominant foliation (sm1+2) on Kythnos record a strong shortening that is perpendicular dicular to the shearing direction resulted in a

is deformed into two open elongated NE-SW– to lm2, with fold axes parallel to this direction strong l-fabric of the footwall rocks below the trending domes, giving the island its distinctive and upright to slightly NW-vergent axial planes. low-angle normal fault. A conglomerate marble, shape (Fig. 1B). At the highest structural levels, The low-angle normal fault on Kythnos is char- which represents a typical marker horizon in the along the culmination of the domes and at the S acterized by abundant kinematic indicators in Cycladic Blueschist unit on Kythnos, but also headland, the schists are intercalated with blue- the marble mylonites and interlayered green- on Kea and Serifos, records a prolate fabric with gray calcite marbles that are deformed into an schists, all of which record a clear top-to-the- a weak foliation and a dominant ENE-WSW–

ultrafi ne-grained, pink to brown impure calcitic SW to -SSW shear sense (Fig. 2E). High-strain trending lineation (lm1), which is parallel to the marble ultramylonite. Millimeter-thin quartz pinch-and-swell shear-band (C′) boudinage of long axes of the white deformed conglomerate layers show evidence for low-temperature grain rotated quartz veins in coarse-grained (0.5 mm) components (Fig. 3A). Because the marble is boundary migration indicative of lower-green- blue marbles is overprinted by localized fi ner- statically recrystallized and does not record schist-facies conditions during deformation. grained milky white shear zones and pervasive any fi ne-grained shear zones, typical for the

A clearly developed stretching lineation (lm2) brittle fracturing and bookshelf boudinage of the extensional deformation along the detachment,

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 29

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

TABLE 2. (U-Th)/He ANALYTICAL DATA FROM ZIRCON, WESTERN CYCLADES Sample Age ± (Ma) U Th Th/U He Mass Ft Std dev (Ma) (ppm) (ppm) (nmol/g) (µg)

Kea KEA0701, 4160758°N, 262048°E, schist KEA0701-2 9.2 0.7 384.9 116.5 0.3 14.74 3.2 0.72 KEA0701-3 7.7 0.6 117.8 103.7 0.9 4.66 7.2 0.79 average 8.4 0.7 251.3 110.1 0.6 9.70 5.2 0.75 1.0 KEA0703, 4165418°N, 265949°E, schist KEA0703-1 7.6 0.6 255.3 156.3 0.6 9.88 15.6 0.83 KEA0703-3 8.7 0.7 347.4 271.2 0.8 15.66 10.7 0.81 average 8.1 0.7 301.4 213.7 0.7 12.77 13.2 0.82 0.8 KEA0707, 4168502°N, 268275°E, schist KEA0707-1 6.9 0.6 673.0 38.6 0.1 18.36 2.7 0.73 KEA0707-2 6.2 0.5 362.3 51.0 0.1 9.29 3.9 0.74 KEA0707-3 6.7 0.5 401.0 45.0 0.1 11.49 6.2 0.77 average 6.6 0.5 478.8 44.9 0.10 13.05 4.3 0.75 0.3 KEA07_10h, 4169315°N, 260942°E, schist KEA0710h-1 2.4 0.2 1084.4 559.6 0.5 11.98 5.4 0.76 KEA0710h-2 7.7 0.6 440.7 208.5 0.5 16.14 7.4 0.79 average 5.1 0.4 762.6 384.1 0.5 14.06 6.4 0.77 3.8 Kythnos 07KO08, 4147573°N, 272067°E, schist 07KO08-1 8.7 0.7 479.5 209.6 0.4 17.68 2.8 0.72 07KO08-2 7.8 0.6 677.3 187.6 0.3 20.87 2.2 0.69 average 8.2 0.7 578.4 198.6 0.4 19.28 2.5 0.70 0.6 07KO18, 4144358°N, 275719°E, schist 07KO18-1 10.8 0.9 242.7 54.1 0.2 11.53 6.4 0.78 07KO18-2 6.2 0.5 159.7 38.7 0.2 4.42 6.9 0.79 07KO18-3 8.3 0.7 407.4 35.8 0.1 15.12 10.2 0.82 average 8.4 0.7 269.9 42.9 0.18 10.36 7.8 0.79 2.3 07KO22, 4137436°N, 268269°E, gneiss 07KO22-1 7.1 0.6 77.7 17.0 0.2 2.52 8.7 0.80 07KO22-2 9.2 0.7 504.8 162.1 0.3 21.03 6.1 0.78 07KO22-3 13.1 1.1 94.9 32.2 0.3 5.78 8.9 0.80 average 9.8 0.8 225.8 70.4 0.29 9.78 7.9 0.79 3.1 07KO25, 4133406°N, 267837°E, schist 07KO25-1 13.3 1.1 0.1 0.9 8.1 0.02 38.6 0.87 07KO25-2 22.3 1.8 0.1 1.1 18.3 0.03 11.5 0.80 average 17.8 1.4 0.1 1.0 13.2 0.03 25.0 0.83 6.4 07KO26, 4146620°N, 270836°E, quartz schist 07KO26-1 9.0 0.7 241.5 124.7 0.5 10.77 13.0 0.82 07KO26-3 13.7 1.1 267.5 154.9 0.6 15.95 2.8 0.71 average 11.4 0.9 254.5 139.8 0.5 13.36 7.9 0.76 3.3

Serifos SER 644, 4112919°N, 272377°E, schist SER 513-1 6.7 0.5 253.8 123.5 0.5 7.90 6.2 0.77 SER 513-2 7.2 0.6 288.1 149.5 0.5 9.10 3.9 0.72 SER 513-3 8.1 0.6 347.7 176.4 0.5 12.24 3.8 0.72 average 7.3 0.6 296.5 149.8 0.50 9.75 4.6 0.74 0.7 SER 70A, 4116391°N, 277541°E, granodiorite SER 70A-1 6.9 0.6 298.2 166.6 0.6 9.22 3.7 0.74 SER 70A-2 6.7 0.5 332.9 131.6 0.4 9.23 2.4 0.71 SER 70A-3 7.6 0.6 470.8 197.3 0.4 15.13 2.8 0.71 average 7.1 0.6 367.3 165.2 0.46 11.19 3.0 0.72 0.5 SER 501, 4116186°N, 279284°E, granodiorite SER 501-1 7.2 0.6 241.7 111.3 0.5 7.97 6.5 0.77 SER 501-2 7.8 0.6 208.9 107.2 0.5 7.74 8.5 0.79 SER 501-3 6.9 0.5 356.7 135.5 0.4 10.75 4.5 0.75 average 7.3 0.6 269.1 118.0 0.45 8.82 6.5 0.77 0.5 SER 514, 4114781°N, 278988°E, granodiorite SER 514-1 7.6 0.6 301.5 120.5 0.4 10.24 5.4 0.76 SER 514-2 7.2 0.6 389.3 187.2 0.5 12.24 3.5 0.73 SER 514-3 6.9 0.6 411.5 130.0 0.3 11.94 3.2 0.72 average 7.3 0.6 367.4 145.9 0.40 11.47 4.0 0.74 0.3 (continued )

30 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

TABLE 2. (U-Th)/He ANALYTICAL DATA FROM ZIRCON, WESTERN CYCLADES (continued ) Sample Age ± (Ma) U Th Th/U He Mass Ft Std dev (Ma) (ppm) (ppm) (nmol/g) (µg) SER 116, 4112163°N, 276148°E, foliated granodiorite SER 116-1 6.1 0.5 821.9 347.7 0.4 21.03 2.6 0.71 SER 116-2 6.0 0.5 977.9 418.7 0.4 24.15 2.2 0.70 SER 116-3 6.3 0.5 512.6 161.5 0.3 13.49 2.9 0.73 average 6.1 0.5 770.8 309.3 0.39 19.56 2.6 0.71 0.1 SER 119, 4112278°N, 274675°E, weakly foliated granodiorite SER 119-1 6.8 0.5 478.5 229.1 0.5 15.33 8.4 0.78 SER 119-2 5.6 0.4 250.1 101.4 0.4 6.62 10.4 0.80 SER 119-3 6.2 0.5 303.5 152.3 0.5 8.92 8.3 0.79 average 6.2 0.5 344.0 160.9 0.46 10.29 9.0 0.79 0.6 SER 340, 4119075°N, 278348°E, schist ZSER 340-1 7.1 0.6 1009.0 94.0 0.1 24.96 1.1 0.63 ZSER 340-2 6.6 0.5 412.3 18.1 0.0 10.29 2.1 0.70 ZSER 340-3 7.5 0.6 766.5 79.2 0.1 22.32 2.5 0.71 average 7.1 0.6 729.3 63.8 0.08 19.19 1.9 0.68 0.4 07SE04, 4120416°N, 279419°E, quartz mica gneiss 07SE04-Zr1 6.8 0.5 142.9 11.8 0.1 3.87 18.1 0.73 07SE04-Zr2 8.1 0.6 784.4 302.9 0.4 27.62 3.7 0.74 07SE04-Zr3 6.0 0.5 518.4 29.0 0.1 13.03 5.1 0.77 average 7.0 0.6 481.9 114.6 0.17 14.84 9.0 0.74 1.1 07SE10, 4111489°N, 270639°E, mica gneiss 07SE10-Zr1 5.4 0.4 521.9 81.3 0.2 11.49 3.4 0.72 07SE10-Zr2 6.0 0.5 1021.9 299.7 0.3 24.75 2.3 0.70 07SE10-Zr3 5.8 0.5 747.4 170.8 0.2 18.01 3.2 0.73 average 5.8 0.5 763.7 183.9 0.23 18.09 3.0 0.72 0.3 07SE12, 4111550°N, 270408°E, mica gneiss 07SE12-Zr1 8.2 0.7 202.4 59.6 0.3 7.07 4.3 0.74 07SE12-Zr2 6.9 0.6 392.5 77.5 0.2 11.04 3.4 0.72 07SE12-Zr3 8.7 0.7 523.2 240.2 0.5 19.63 3.1 0.73 average 7.9 0.6 372.7 125.8 0.32 12.58 3.6 0.73 0.9 07SE13, 4111610°N, 270404°E, mica gneiss 07SE13-Zr1 9.1 0.7 553.3 344.0 0.6 22.64 3.1 0.73 07SE13-Zr2 6.7 0.5 596.2 97.9 0.2 15.82 2.4 0.71 07SE13-Zr3 6.6 0.5 330.8 186.0 0.6 19.82 2.8 0.71 average 7.5 0.6 493.5 209.3 0.45 19.43 2.8 0.72 1.4

the deformation is considered to belong to the Miocene cooling, spanning 10 m.y. from 18 Ma eastern part of the island, the structurally high- Eocene exhumation. to as young as 8 Ma. Older ages are clustered in est branch of a low-angle normal fault system Exhumation of the footwall rocks is associ- the southern portion of the island near the detach- cuts the roof of the granodiorite pluton, record- ated with a strong greenschist-facies overprint ment at Aghios Dimitrios; similar to Kea, ZHe ing progressive deformation of the undeformed throughout the island. White mica K-Ar dat- ages are younger then AHe ages (Tables 1 and 2). intrusion at lower structural levels to mylonitic ing constrains the greenschist-facies conditions deformation within the fault zone. The higher to ca. 26–20 Ma (Schliestedt et al., 1994). Our Serifos temperatures achieved during deformation, as new 40Ar/39Ar white mica ages from footwall indicated by recrystallized K-feldspar (Tullis and schists are consistently younger, yielding ages The geology of Serifos (Fig. 1C) is domi- Yund, 1987), have been interpreted as the result of ca. 20 Ma, coeval with the tectonism on Kea nated by an I-type granodiorite pluton that was of the synkinematic intrusion of the granodio- (Fig. 4; Table A1 [see footnote 1]). The spectra intruded into shallow crustal levels above the rite into an extensional shear zone (Tschegg and possess a slight convex-upward shape and early brittle-ductile transition zone (Marinos, 1951; Grasemann, 2009). Along its western margin,

Miocene apparent ages. The general hump-shape Stouraiti et al., 2010). Intrusion ages of the the pluton discordantly crosscuts the sm1+2 folia- of these spectra is defi ned by heating incre- pluton and its associated dikes, which invaded tion of the metamorphic host rocks, composed ments that initially monotonically increase in W-E–striking brittle high-angle faults, lie of three distinct units, all of which are separated age, approach a quasi-plateau (20%–40% of between 11.6 and 9.5 Ma (Altherr et al., 1982; by two individual branches of the low-angle nor- total 39Ar) during moderate experimental heat- Iglseder et al., 2009). In this work, we report mal fault system with similar top-to-the-SSE– ing stages, and decline again as the fi nal heating new 40Ar/39Ar white mica ages from rocks adja- directed kinematics: stages are approached. The K/Ca ratios associated cent to the pluton, within its thermal aureole, (1) The tectonically lowermost unit consists with these increments closely mimic the initial which yield ca. 9 Ma ages, and a few older ages of massive >200-m-thick mylonitic gneisses and rising trend while maintaining reasonably high (ca. 13–11 Ma) on mylonites away from the con- schists, which are under- and overlain by calcite/ values for the remainder of gas release. The fi nal tact (Fig. 4; Table A1 [see footnote 1]). These dolomite marble mylonites having a dominant 40 39 increment is typically anomalously old, with an youngest Ar/ Ar ages of the three islands are W-E to WSW-ENE striking lineation (lm1) and antithetic relationship with the K/Ca ratio. Zircon also the most well-behaved spectra, with 90% of a clear top-to-the-W shear sense (Fig. 3C). The and apatite (U-Th)/He ages record middle to late the 39Ar released defi ning the plateau ages. In the mylonitic gneiss was deformed at temperatures

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 31

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

A B R~Rxyy ~ 55-10-1-10 c‘c R~Ryyz ~ 22-3-3-3

s c

SSNN WSWlm1 EN ENENE C D

ENEENENE WSW WSWSW WWSWSWSW ENE ENENE

E AnkeritizedAnkeri zed protocataclasitespprotocataclasites F lfm ofo thethe UpperUppere CycladicCyc ad c ununit FoFFoliatedo eded s (proto)cataclastic(p(prorotoo)c)caatacc asst c sfm c′ marbles/schistsmam rbrb es//scch s axaxplxplp axpl ovoverprintedvererprpr nted byby lfb SCSSCCCC′ fabricabbric

MarbleMaM rbb e uultramylonitesrar myy on tees wwiththh Unfoliated/chaoticUnU foo tedd//cchah o c NNE-SSW-trendingNNE--SSSSW-W trrennd ng lineationneae t ono cataclasites/clayccattac as ese /c/c y gougegogouguge CBUCBC U withw thh WWSW-ENE-trendingSWW-EENENE-trreendn ng lineationnenea on sfmm WWEE ESEESESE lfmm WNWWNWNW

Figure 3. (A) Cataclasites along the detachment above the marble mylonites on Kythnos (4131485N, 266483E). Dolomitic layers (brown color) deform by dilational fracturing and pervasive precipitation of calcite veins. Calcite (bluish/gray) deforms by dissolution-precipitation creep forming SCC′ structures. (B) Typical conglomerate marble on the east coast of Kythnos (4142051N, 275684E). The prolate fabric has a weak foliation (062°/23°) but is dominated

by the lineation 061°/22°, which is parallel to the long axes of the white deformed conglomerate components (lm1). (C) Mylonitic gneiss of the Cycladic basement in S Serifos (4110382N, 276654E). The mylonitic foliation dips gently toward the S and developed a NNE-SSW–striking stretching linea- tion. Quartz veins have been rotated into the shear direction forming asymmetric pinch-and-swell boudinage indicating top-to-the-WSW kinematics. (D) Conglomerate marble on Serifos that is 10 m below the low-angle normal fault at Platy Yialos (4120490N, 279330E). The prolate-shaped components record the long axis, which strikes 065°–245°, and they have σ-type geometries with clear stair-stepping, indicating WSW-directed shear. (E) Knife-sharp brittle low-angle normal fault (270°/10°) on Kavos Kiklopas (41.12546N, 270371E). Note that the contact marked by decimeter-thick ultracataclasites is localized exactly above the marble ultramylonites. CBU—Cycladic Blueschist unit. (F) The detachment plane exposed on N Serifos (4120542N, 279948E)

forms a knife-sharp slickenside (sfm) localizing above a several-meter-thick marble ultramylonite. The slickenlines (lfm) are parallel to lm2 in the marble ultramylonites trending NNE-SSW. The foliated protocataclasites above the detachment are deformed into upright detachment folds with fold axial planes (axpl) also parallel to the shear direction. The cores of the detachment folds are fi lled with clay-rich, chaotic ultracataclasites and fault gouges.

32 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

above ~450 °C and exhibits striped-gneiss micro- K/Ca K/Ca structures preserving coarse-grained (>1 mm) 40 quartz grains with straight grain boundaries and KEA 06/19 schist KEAKEA 08/26 calc-silicate schist KEA dynamically recrystallized K-feldspar (Pass- 30 chier and Trouw, 2005; Tullis and Yund, 1987). 19.98 ± 0.73 Ma 19.49 ± 0.73 Ma Some porphyroclasts record magmatic relics, 20 T E F G H L MNO P S such as perthitic K-feldspar and sharply zoned I K K L Q R D J C H I J plagioclase. Secondary ion mass spectrometry 10 B G (SIMS) depth-profi ling U-Pb geochronology on

Apparent age (Ma) Tg = 18.61 ± 0.03 Ma Tg = 19.17 ± 0.07 Ma zircons separated from this unit yields an age 0 of ca. 40 Ma, with rare earth element (REE) signatures and Th/U values indicative of (high- K/Ca K/Ca pressure?) metamorphism; cores of the zircons 40 yield a mixed age spectrum with age populations KEA 06/14 calc-silicate schist KEAKEA 06/24 schist KEA between Carboniferous and Triassic (Schneider 30 18.86 ± 0.1 17.04 ± 0.41 Ma et al., 2011). Analogous to the lithotypes below 20 the Upper Tectonic unit on Attica (equivalent to A G I 1 Ma F H E FGH KL the Cycladic Blueschist unit; Baziotis and Mpos- E A D I J C D C , 2011) and the structural level of the granodi- 10 B B

Apparent age (Ma) orite intrusion into the Lower Tectonic unit (i.e., Tg = 17.64 ± 0.04 Ma Tg = 16.39 ± 0.03 Ma 0 the Basal unit; Skarpelis et al., 2008), these rocks are suggested to represent a part of the Basal unit. K/Ca Toward higher structural levels, the gneisses and K/Ca 40 the marble mylonites are increasingly overprinted M/K 101 schist KEA by a noncoaxial, greenschist-facies, low-angle 07K0-01 schist KYTHNOS 30 normal fault with NNE-SSW–oriented stretch- 18.50 ± 0.53 Ma 19.92 ± 0.05 Ma ing lineation (lm2) and SSW-directed shear sense. 20 K The fault contact to the overlying Cycladic Blue- F G I M E H C D E F G H J K L D I J B schist unit is marked by a decimeter-thick zone C B A 10 A of talc and marble cataclasites and a several-tens-

Apparent age (Ma) Tg = 18.62 ± 0.06 Ma Tg = 20.1 ± 0.1 Ma of-meters-thick zone of gneisses/amphibolites 0 protocataclasites, derived from the hanging wall (Grasemann and Tschegg, 2011). This branch of the low-angle normal fault is cut by the granodio- K/Ca K/Ca rite intrusion, which was also the likely source for 40 07K0-16 schist KYTHNOS07K0-24 schist KYTHNOS fl uid infi ltration along the fault zone. 30 (2) The overlying Cycladic Blueschist unit 18.77 ± 0.09 Ma 18.9 ± 2.4 Ma is characterized by basal amphibolites and 20 greenschist sequences intercalated with mar- H C D E F G I J L D E B K C F G H bles, which dominate northern Serifos. Relic B 10 A occurrences of glaucophane suggest that these A Apparent age (Ma) Tg = 18.2 ± 0.1 Ma Tg = 17.2 ± 0.1 Ma rocks have experienced metamorphism related 0 to the widespread high-pressure event typical for the Cycladic Blueschist unit (Salemink and Schuiling, 1987). Deformation is characterized K/Ca K/Ca 40 by mesoscopic folds with subhorizontal axial 07K0-23 schist KYTHNOS07K0-04 schist KYTHNOS planes and fold axes parallel to the WSW-ENE–

30 trending stretching lineation (lm1). 20.7 ± 0.1 Ma 21.2 ± 0.1 Ma In the northern part of Serifos, 10 m below the 20 E F G D F G E H I L low-angle normal fault at Platy Yialos, the con- C C D J K B B glomerate marble marker horizon is exposed. The 10 A A marbles have an almost horizontal foliation, and

Apparent age (Ma) Tg = 17.7 ± 0.1 Ma Tg = 19.0 ± 0.1 Ma 0 the prolate-shaped components record a long axis 0 20 40 60 80 100 0 20406080100that trends 065°–245°. In a section parallel to the 39 Cumulative %39 Ar released Cumulative % Ar released lineation and perpendicular to the foliation, the components have σ-type geometries with clear Figure 4. 40Ar/39Ar white mica age spectra from Kea, Kythnos, and Serifos. Tg—total-gas integrated stair-stepping indicating WSW-directed shear age. K/Ca scale: 1–1000 for Kea and Serifos and 0.1–1000 for Kythnos. See Figure 1 for sample loca- (Fig. 3D). Interestingly, the conglomerate marble tions, Table A1 for analytical details (see text footnote 1), and the GSA Data Repository for analyti- is almost unaffected by the SSW-directed shear of cal methodology (see text footnote 1) (Continued on following page). the nearby low-angle normal fault, confi rming the

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 33

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

2002). The low-grade mylonites are cut by knife- sharp slickenside planes (Figs. 3E and 3F), above K/Ca K/Ca 40 which centimeter-thick multistage generations of SERIFOS unfoliated and foliated ultracataclasites record SERF 567 mylonite SERF 53 gneiss SERIFOS 30 SSW-directed shearing in the brittle-ductile and brittle regime. Both brittle deformation and duc- 20 tile deformation record folds with upright axial 8.43 ± 0.05 Ma 8.93 ± 0.05 Ma planes and NNE-SSW–striking fold axes, indica- 10 tive of a strong shortening component perpendic- D EFGH IJK M CD E FG HI JKLM Apparent age (Ma) B C L B ular to the shear (extension) direction (Fig. 3F). A Tg = 8.31 ± 0.03 Ma Tg = 8.84 ± 0.03 Ma 0 Interestingly, the low-angle normal fault interacts with, and postdates, sets of WNW-ESE–striking, conjugate high-angle normal faults, which were K/Ca K/Ca 40 also exploited during emplacement of the grano- SERF 107 mylonite SERIFOSSERF 358 mylonite SERIFOS diorite intrusion-related dikes (Grasemann and 30 Petrakakis, 2007; Iglseder et al., 2009), indicat- ing deformation during NNE-SSW extension of 20 the crust under subvertical maximum principal 9.33 ± 0.16 Ma 8.89 ± 0.05 Ma stresses. Notably, no 40Ar/39Ar age gradient is 10 L observed across the fault zone, and, except for E F G M L Apparent age (Ma) B C D H I JK B C D EFGHI J K younger ages close to the contact of the grano- Tg = 9.17 ± 0.06 Ma A Tg = 8.86 ± 0.04 Ma 0 diorite pluton, only Eocene ages are preserved (Schneider et al., 2011). All zircon and apatite (U-Th)/He ages from the Cycladic Blueschist unit K/Ca K/Ca range from 8 to 5 Ma and record late Miocene 40 cooling (Tables 1 and 2), in accord with published SERF 428 mylonite SERIFOSSERF 571 mylonite SERIFOS 30 fi ssion-track data (Hejl et al., 2002; Brichau et al., M L 2010). These observations are indicative of a tem- 12.9 ± 0.4 Ma 20 perature of ~300 °C during ductile subhorizontal 8.45 ± 0.02 Ma movement of the low-angle normal fault, consis- F K D G J 10 B C E HI tent with the observed deformation mechanisms DE F GHIJK Apparent age (Ma) A in calcite and quartz (Passchier and Trouw, 2005). Tg = 15.01 ± 0.04 Ma Tg = 8.40 ± 0.03 Ma 0 (3) Remnants of the Upper Cycladic unit are 0 20 40 60 80 exposed above the cataclastic fault core of the branch of the low-angle normal fault separating K/Ca the Cycladic Blueschist unit in the footwall. The 40 rocks consist mainly of marble-dominated pro- SER08/01 marble mylonite SERIFOS 30 tocataclastic lithologies that are strongly ankeri- Figure 4 (continued). tized by massive fl uid infi ltration (Fig. 3E). In 20 the SW peninsula of Serifos, a highly altered 11.2 ± 0.5 Ma several-tens-of-meters-thick serpentinite lens 10 P associated with talc schists is exposed. Since KMNO R S T Apparent age (Ma) Q only remnants of the Upper Cycladic unit are Tg = 13.15 ± 0.11 Ma 0 preserved, and these marble-rich protocata- 020406080 100 clasites are strongly altered, it was diffi cult to Cumulative %39 Ar Released separate suitable material for geochronological dating. However, whereas most apatite (U-Th)/ He analyses from Serifos yielded late Miocene observation of extremely localized deformation Ductile deformation is extremely localized within ages, one sample from the Upper Cycladic unit of the ductile-brittle transition zone. The 40Ar/39Ar only a few-meter-thick ultrafi ne-grained marble at Platy Yialos and another at Kavos Kiklopas analyses of white mica of the Cycladic Blueschist mylonite (Fig. 3E). The typical stretching linea- gave 12–15 Ma ages (Fig. 1; Table 2).

unit preserve ages of 38–35 Ma (Schneider et al., tion associated with this event (lm2) trends NNE- 2011), demonstrating that the WSW-trending lin- SSW and has been interpreted as the result of DISCUSSION eations are the result of an earlier phase of defor- Miocene crustal-scale extension (Grasemann and mation within the western Cyclades, predating Petrakakis, 2007; Iglseder et al., 2009; Brichau et Signifi cance and Timing of the West the NNE-SSW–directed extension. The tectonic al., 2010). Quartz in the marble mylonites records Cycladic Detachment System contact to the overlying Upper Cycladic unit is microstructural evidence for dislocation glide and another branch of the low-angle normal fault, brittle fracturing but no evidence for dislocation Our tectonic and thermochronological anal- which arches over the island and consistently creep, suggesting deformation conditions within yses of the western Cyclades presented here, records a top-to-the-SSW–directed shear sense. the brittle-ductile transition zone (e.g., Stipp et al., combined with previous work (Hejl et al., 2002;

34 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

Grasemann and Petrakakis, 2007; Iglseder et that ductile movement along the low-angle indicative of total displacements on the order of al., 2009, 2011; Tschegg and Grasemann, 2009; normal fault was active in the early Miocene. several tens of kilometers (Jolivet et al., 2010). Brichau et al., 2010; Schneider et al., 2011), allow Although the onset of backarc extension has Quantitative constraints of natural fault systems us to consider several detachments observed on been suggested to have started between 35 and demonstrate that faults with strike lengths on the the islands of Kea, Kythnos, and Serifos. Each of 30 Ma (Jolivet and Brun, 2010), a comparison order of 100 km would be required to achieve the low-angle normal faults bears many similari- of Aegean-wide extensional structures revealed maximum displacements on the order of tens of ties in terms of their tectonometamorphic evo- that there was an important phase of extensional kilometers (Walsh and Watterson, 1988; Cowie lution in exhuming the Cycladic Blueschist unit deformation starting around 23 Ma (e.g., Altherr and Scholz, 1992; Dawers et al., 1993). In order with Miocene top-to-the-SW to -SSW–directed et al., 1982; Wijbrans and McDougall, 1988; to accommodate the displacements necessary to kinematics, and an arching of the ductile to brit- Bröcker et al., 1993). The thermochronological exhume these rocks from such depths, a more tle shear zone over the islands that defi nes the and structural data are supported by the observa- laterally extensive fault system is necessary. carapace of the exhumed domes. These observa- tion that the oldest sediments in the extensional We therefore propose that the low-angle nor- tions are in marked contrast to the long-standing basins of the Aegean are also early Miocene in mal faults on Kea, Kythnos, and Serifos form view that extension in the northern and western age (Buttner and Kowalczyk, 1978), suggest- a mechanically linked single crustal-scale struc- Cyclades is dominated by a single NE-directed ing that a Miocene regional extension resulted ture, which we term the West Cyclades detach- displacement or that top-to-the-S kinematics in the opening of the basin. Addi- ment system (Fig. 5). represent only local brittle-ductile deformation tionally, deposition of shallow-marine detrital (e.g., Avigad and Garfunkel, 1989; Buick, 1991; sediments on the Upper Cycladic unit started Hanging Wall of the West Cycladic Faure et al., 1991; Lee and Lister, 1992; Gautier around 23 Ma (Angelier et al., 1978; Sánchez- Detachment System et al., 1993; Gautier and Brun, 1994; Jolivet Gómez et al., 2002; Kuhlemann et al., 2004). and Patriat, 1999; Vanderhaeghe, 2004; Mehl Based on thermochronological data, extensional Due to the level of erosion, only remnants of et al., 2007; Jolivet et al., 2010). Whether or not faulting on Serifos was active between 15 and the hanging wall of the West Cycladic detach- extension in the western Aegean crust has been 6 Ma (Iglseder et al., 2009; Brichau et al., 2010; ment system (i.e., Upper Cycladic unit) are pre- accommodated by bivergent low-angle normal this study). Further, an unambiguous timing served on Kea, Kythnos, and Serifos. Addition- faults depends on the regional importance given constraint for the extensional displacement is ally, these remnants have been strongly altered to the top-to-the-S kinematics and their tim- the emplacement of the Serifos granodiorite by infi ltration of fl uid along the low-angle nor- ing with respect to movement along the North between 11.6 and 9.5 Ma (Iglseder et al., 2009), mal faults. Nevertheless, the serpentinite in SW Cycladic detachment system. which crosscuts a structurally deeper branch of Serifos and some remnants of the unmetamor- It is evident that the low-angle normal faults the low-angle normal fault between the Basal phosed hanging wall on Kea suggest that these in the western Cyclades, similar to the North unit and the Cycladic Blueschist unit, but its roof rocks are not part of the Cycladic Blueschist Cycladic detachment system, were active at very is cut by a branch of the low-angle normal fault, unit. The rocks in the hanging wall of the West shallow dip angles, because they mechanically postdating the intrusion. Although the post- Cycladic detachment system should be part of interacted with conjugate high-angle normal intrusive displacement has been suggested to the Upper Cycladic unit representing the lateral faults and record brittle-ductile vertical vein exceed 1.5 km (Tschegg and Grasemann, 2009), continuation of the Pelagonian unit on the Greek formation typical for a vertical maximum prin- the exhumation of rocks from midcrustal levels mainland (Bonneau, 1982). Interestingly, on the ciple stress direction (Mehl et al., 2005, 2007; along very shallow-dipping normal faults is island of , situated 40 km SSW of Serifos Lecomte et al., 2010; Collettini, 2011). Whereas ductile extensional deformation is pervasive in the footwall of the low-angle normal fault on A ႑ 25 km Quaternary volcanics Kea and partly also on Kythnos, the low-angle A ႑ NCDS

႑ Miocene granitoids ႑ ႑ Figure 5. (A) Geological map normal fault between the Cycladic Blueschist ႑ N

Attica ႑ Pelagonian unit of the Cyclades. Locations

unit and the Upper Cycladic unit is extremely ႑ Cycladic blueschist of the main sedimentary Kea ႑ localized along the brittle-ductile transition zone ႑ ႑ ႑ ႑ basins and high-angle faults ႑ Cycladic basement rocks on Serifos. Therefore, the earlier ENE- ႑ are modifi ed after Mascle ႑ ႑ ႑ ႑ ႑ ႑ ႑ ႑ ႑ Low-angle fault and Martin (1990). Miocene WSW–oriented lineation is completely over- Kythnos ႑ kinematic directions are printed on Kea and best preserved on Serifos, ႑ High-angle fault 40 39 shown by white arrows, ႑ B where ca. 38–35 Ma white mica Ar/ Ar ages ႑ Sedimentary basin A′ ႑ which indicate the direc- ႑

(Schneider et al., 2011) record exhumation from Serifos ႑ ႑ ႑ ႑ ႑ tion of tectonic movement;

႑ ႑

႑ ႑ ႑ ႑

႑ ႑ earlier tectonometamorphic conditions. The ႑ ႑ ႑ Eocene kinematic directions

႑ ႑ ႑ ႑

Eocene ENE-WSW–trending lineations could 37ºN ႑ ႑ are shown by black arrows be evidence for the basal thrust of a high-pressure WCDS (after Huet et al., 2009, and extrusion wedge that brought up the Cycladic references cited therein) and Blueschist unit during subduction, which is this work). (B) Schematic crustal section illustrating complemented by top-to-the-NE normal sense Milos the bivergent nature of Mio- 24ºE ′ displacement on Sifnos and Syros (Trotet et al., B cene extension in the west- 2001) illustrating the top of the wedge (Ring et Kea Andros B A′ A ern Cyclades. WCDS—West NCDS al., 2007, 2011; Huet et al., 2009). Milos Serifos Cycladic detachment sys- The exact onset of latest extension is unclear, B′‘ B tem; NCDS—North Cycladic 40 39 detachment system. but ca. 20–17 Ma Ar/ Ar ages on fabric form- WCDS 25 km ing white mica from Kea and Kythnos suggest

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 35

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

and in the hanging wall of the West Cycladic system is probably found in Attica (mainland fault on Serifos. In confl ict with these interpre- detachment system, a Mesozoic metamorphic Greece), where several low-angle normal faults tations, Huet et al. (2009) has shown that top- basement and an Upper Miocene–Lower Plio- at different structural levels have been reported to-the-S deformation on Ios is recorded in shear cene fossiliferous transgressive marine sedi- (e.g., Krohe et al., 2010). Similar to Serifos, bands and pressure shadows associated with mentary sequence are exposed (Fytikas, 1977); close to the SE end of the West Cycladic detach- blueschist-facies minerals and proposed that the therefore, Milos cannot be part of the Cycladic ment system, a late-tectonic granodiorite pluton Cycladic Blueschist unit–Cycladic Basement Blueschist unit. In fact, on Milos, the fi rst marine intruded between 11.9 and 8.8 Ma (Skarpelis et unit contact (i.e., South Cycladic shear zone) sediments were deposited on the hanging wall al., 2008; Liati et al., 2009) into the low-angle is not a normal fault but a thrust, active in the of the West Cycladic detachment system, while normal fault contact between the Lower Tec- subduction zone at high-pressure–low-temper- the rocks on Serifos were exhuming and cooling tonic unit (equivalent of the Basal unit; Baziotis ature conditions. The data of the present paper from midcrustal level in the footwall position of et al., 2009) and the Upper Tectonic unit (equiv- do not directly contribute to the long-standing the West Cycladic detachment system. alent of the Cycladic Blueschist unit; Baziotis debate about the South Cycladic shear zone. and Mposkos, 2011), pinning the latest stage However, the existing data from the western Possible Lateral Continuation of the West of movement along the West Cycladic detach- Cyclades strongly support a termination of the Cycladic Detachment System ment system. Similar to the tectonic history on West Cycladic detachment system to the SE on Serifos, the fi nal extensional movements along Sifnos, where only brittle top-to-the-SSW dis- If we accept that the low-angle normal the low-angle normal faults took place at rather placement is recorded, and no Upper Cycladic faults exposed in the western Cyclades are shallow depths, postdating the intrusion (Skar- unit is juxtaposed in the hanging wall. indeed linked, the maximum displacement pelis et al., 2008). Abundant shear sense indica- Our work does provide strong evidence that along the West Cycladic detachment system tors in both ductile marble mylonites and brittle Miocene tectonism in the western Cyclades was would probably be along the approximate polyphase cataclasites between the Lower and dominated by top-to-the-SW extension, and that center of the detachment, which is on Kea, the Upper Tectonic units record SSW-directed at least the western and northern parts of the where the thickest marble ultramylonites kinematics similar to the shear sense of the West Cyclades represent a bivergent extensional oro- are present (up to several tens of meters). Cycladic detachment system (Figs. 6C and 6D). gen. Most geodynamic numerical models (e.g., Additionally, ductile deformation associ- Chéry et al., 1992; Bassi et al., 1993) are either ated with extension is much more distrib- Bivergent Extension in the Aegean intrinsically symmetric or predict symmetric uted in the footwall on Kea, suggesting a extension, and only models with inherited inho- relatively deeper exposed level of the Mio- We suggest that the low-angle normal fault mogeneities developed asymmetric extension cene low-angle normal fault with respect to on Serifos links with S-directed low-angle nor- (e.g., Govers and Wortel, 1993; Tirel et al., 2008). the fault system on Serifos. It is on Serifos mal faults on the adjacent islands of Kythnos More complex numerical models considering that a branch of the low-angle normal fault, and Kea, forming the West Cycladic detach- mechanical feedback processes have shown a which separates the Cycladic Blueschist unit ment system, which, together with the North strong sensitivity of rift mode to various param- from the Upper Cycladic unit, characterized Cycladic detachment system, accommodated eters like the strength of the lithosphere, strain by the marble ultramylonites, reaches only a Miocene extension in the Aegean. The oldest softening processes, crust-mantle coupling, and few meters in thickness. The SE termination documentation of a low-angle normal fault in extension velocities (Huismans and Beaumont, of the fault could be located at the southern the Aegean region with top-to-the-S kinemat- 2002; Huet et al., 2011). Although fi eld data end of Sifnos, where discrete S-directed, sub- ics comes from the South Cyclades shear zone, have shown that within the metamorphic core horizontal, meter-long brittle-ductile shear exposed on Ios (Lister et al., 1984). The island complexes, the dome-shaped foliation envelope bands have been reported (Figs. 6A and 6B; of Ios notably preserves both S-directed and can be associated with opposite senses of shear Weil et al., 2010). Although Sifnos has been N-directed shear sense; the latter kinematics along opposing limbs of the dome (Gautier and considered to record penetrative top-to-the- are preserved in the northern part of the island Brun, 1994), the majority of metamorphic cores NE sense of shear (Avigad, 1993; Trotet and are characterized by greenschist-facies and show a uniform sense of shear from one limb et al., 2001), the recent work of Ring et al. brittle conditions (Huet et al., 2009). In order to to the other, with fi nite strain intensities usu- (2011) suggested the existence of a Sifnos reconcile the bivergence of Ios and the well-con- ally higher in the low-angle normal fault zones detachment with top-to-the-SSW kinematics strained top-to-the-N shear sense of the nearby (Davis, 1983; Lister and Davis, 1989). that operated under brittle conditions with Naxos metamorphic core complex (Urai et al., It is important to note that the North Cycladic only minor displacement. Low-temperature 1990; Buick, 1991; Gautier et al., 1993), sev- detachment system and the West Cycladic thermochronological data suggest that move- eral authors suggested either a complex switch detachment system represent individual low- ment along the Sifnos detachment largely of the shear sense or a bivergent exhumation of angle normal fault systems with distinct tec- terminated between 13 and 10 Ma (Ring et Ios during the Miocene (Vandenberg and Lister, tonometamorphic evolutions, which form both al., 2011). These observations together with 1996; Forster and Lister, 2009; Thomson et al., a number of discrete domes and uniform sense the fact that the Sifnos detachment does not 2009). Such symmetrically arranged detach- of shear across the dome. Therefore, a discus- juxtapose Cycladic Blueschist unit against ment systems, which defi ne a metamorphic core sion about symmetric and asymmetric extension the Upper Cycladic unit, but operated within complex and establish a bivergent continental should involve both the scale and the three- the Cycladic Blueschist unit with only minor breakaway zone, have been also suggested for dimensionality of the structures. In three dimen- displacement, corroborate our suggestion that the central Menderes in the Anatolide belt of sions, different fault segments with various spa- Sifnos may represent the SE termination of western (Hetzel et al., 1995; Gessner et tial sizes and orientations may accommodate the West Cycladic detachment system. al., 2001). Recently, Ring et al. (2011) suggested crustal extension, and therefore bivergent crustal On the other end of the fault system, the NW that the South Cyclades shear zone on Ios could extension in the western and northern Cyclades continuation of the West Cycladic detachment be laterally linked with the low-angle normal does not necessarily require bivergent exten-

36 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

A B

R′ cacataclasitetatac as R′ R′ R′

FiFFig.g.g 66B SSSSSWSW NNENNNNE SSSSSWSW NNENNNNE C D

4 ccm

NNENNNNE SSSSSWSW SSSSSWSW NNENNNE

Figure 6. (A–B) Several-tens-of-meters-long, horizontal, brittle-ductile shear zones in SW Sifnos (4086783N, 295786E). These shear zones record only a few meters offset within the Cycladic Blueschist unit and have a clear top-to-the-SSW shear sense with evidences of ductile fault drag, dissolution/precipitation creep, and cataclastic deformation. R′ indicates synthetic Riedel faults (~200°/25°). (C) Marble mylonites within the Upper Marble of the Lower Tectonic unit (equivalent of the Basal unit), with a strong NNE-SSW–striking stretching lineation and top-to-the-SSW shear sense north of Lavrion (Theater Thorikos, 4181758N, 240418E), separated by a zone of cataclastic deformation the Upper Tectonic unit (equivalent of the Cycladic Blue- schist unit) form the hanging wall. (D) Multiple generations of foliated and unfoliated cataclasites from a low-angle normal fault contact between the Upper and the Lower Tectonic units with SSW-dipping synthetic high-angle brittle faults (Attica, 4186230N, 241254E).

sion of the whole Aegean block. With some (2) The sense of shear from these Miocene along an Extended Lithosphere). We also thank notable exceptions (e.g., Brichau et al., 2006, structures consistently is indicative of SW- IGME, the Institute for Mining and Explora- 2008), the geochronological data from the west- SSW–directed extension. tion in , for providing permission and ern Cyclades and from many other parts of the (3) We propose the mechanical linking of technical support. Stimulating discussions with Aegean currently do not allow a precise tempo- these structures, resulting in the West Cyclades K. Petrakakis, H. Rice, E. Draganits, B. Huet, ral resolution to determine the period(s) during detachment system, which was broadly syn- N. Mancktelow, C. Tschegg, and K. Soukis are which the individual low-angle normal fault seg- chronous with the North Cyclades detachment greatly acknowledged. L. Jolivet and G. Axen ments were active in the Miocene, and therefore system, the timing of which is proposed to be provided important comments to an earlier a temporal and spatial jump of the activity cannot during the peak backarc extension and detach- version of this manuscript. Two anonymous be excluded based on the presented data. Future ment activity in the Cyclades. reviews and handling of the manuscript by tectonic and quantitative models for the region (4) At least in the western and northern R.M. Russo, are gratefully acknowledged. now need to be able to resolve higher temporal Cyclades, these roughly symmetrically arranged resolution and three-dimensional mechanics. detachment systems defi ne bivergent crustal- REFERENCES CITED scale boudinage. CONCLUSIONS Altherr, R., Schliestedt, M., Okrusch, M., Seidel, E., Kreuzer, ACKNOWLEDGMENTS H., Harre, W., Lenz, H., Wendt, I., and Wagner, G.A., 1979, Geochronology of high pressure rocks on Sifnos (1) We have identifi ed hitherto unrecognized (Cyclades, Greece): Contributions to Mineralogy and concomitant Miocene low-angle normal fault We thank the Austrian Science Fund FWF Petrology, v. 70, p. 245–255, doi:10.1007/BF00375354. Altherr, R., Kreuzer, H., Wendt, I., Lenz, H., Wagner, G.A., systems on three islands of the western Cyclades. (grant number: P18823-N19) for supporting Keller, J., Harre, W., and Hohnsdorf, A., 1982, A late project ACCEL (Aegean Core Complexes Oligocene/early Miocene high temperature belt in the

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 37

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 GRASEMANN ET AL.

Attic-Cycladic crystalline complex (SE Pelagonian, Serifos detachment, Aegean Sea, Greece: International 614, doi:10.1130/0091-7613(2001)029<0611:AABRHD>2.0 Greece): Geologisches Jahrbuch, v. E23, p. 97–164. Journal of Earth Sciences, v. 99, p. 379–393, doi:10.1007/ .CO;2. Altherr, R., Kreuzer, H., Lenz, H., Wendt, I., Harre, W., and s00531-008-0386-0. Gessner, K., Wijns, C., and Moresi, L., 2007, Signifi cance of Dürr, S., 1994, Further evidence for a Late Cretaceous Bröcker, M., and Franz, L., 1998, Rb/Sr isotope studies on strain localization in the lower crust for structural evo- low-pressure/high-temperature terrane in the Cyclades, Tinos Island (Cyclades, Greece): Additional time lution and thermal history of metamorphic core com- Greece: Petrology and geochronology of crystalline constraints for metamorphism, extent of infi ltration- plexes: Tectonics, v. 26, doi:10.1029/2004TC001768. rocks from the islands of Donoussa and Ikaria: Chemie controlled overprinting and deformational activity: Govers, R., and Wortel, M.J.R., 1993, Initiation of asymmetric der Erde, v. 54, p. 319–328. Geological Magazine, v. 135, p. 369–382, doi:10.1017/ extension in continental lithosphere: Tectonophysics, Angelier, J., Glacon, G., and Müller, C., 1978, Sur la presence S0016756898008681. v. 223, p. 75–96, doi:10.1016/0040-1951(93)90159-H. et la position tectonique du Miocene inferieur marin Bröcker, M., Kreuzer, H., Matthews, A., and Okrusch, M., Grasemann, B., and Petrakakis, K., 2007, Evolution of the Seri- dans l’archipel de Naxos (Cyclades, Greece): Comptes 1993, 40Ar/39Ar and oxygen isotope studies of polymeta- fos metamorphic core complex, in Lister, G., Forster, Rendus de l’Academie des Sciences de Paris, v. 286, morphism from Tinos Island, Cycladic blueschist belt, M., and Ring, U., eds., Inside the Aegean Metamorphic p. 21–24. Greece: Journal of Metamorphic Geology, v. 11, p. 223– Core Complexes: Journal of the Virtual Explorer, v. 27, Avigad, D., 1993, Tectonic juxtaposition of blueschists and 240, doi:10.1111/j.1525-1314.1993.tb00144.x. doi:10.3809/jvirtex.2007.00170. greenschists in Sifnos Island (Aegean Sea): Implica- Bröcker, M., Bieling, D., Hacker, B., and Gans, P., 2004, Grasemann, B., and Tschegg, C., 2011, Localization of deforma- tions for the structure of the Cycladic blueschist belt: High-Si phengite records the time of greenschist tion triggered by chemo-mechanical feedback processes: Journal of Structural Geology, v. 15, p. 1459–1469, facies overprinting: Implications for models suggest- Geological Society of America Bulletin (in press). doi:10.1016/0191-8141(93)90006-V. ing mega-detachments in the Aegean Sea: Journal of Hejl, E., Riedl, H., and Weingartner, H., 2002, Post-plutonic Avigad, D., and Garfunkel, Z., 1989, Low-angle faults above Metamorphic Geology, v. 22, p. 427–442, doi:10.1111 / unroofi ng and morphogenesis of the Attic-Cycladic and below a blueschist belt, Tinos Island, Cyclades, j.1525-1314.2004.00524.x. complex (Aegean, Greece): Tectonophysics, v. 349, Greece: Terra Nova, v. 1, p. 182–187, doi:10.1111/j.1365 Buick, I.S., 1991, Mylonitic fabric development on Naxos: p. 37–56, doi:10.1016/S0040-1951(02)00045-8. -3121.1989.tb00350.x. Journal of Structural Geology, v. 13, p. 643–655, Hetzel, R., Passchier, C., Ring, U., and Dora, O., 1995, Biver- Avigad, D., Garfunkel, Z., Jolivet, L., and Azañón, J.M., 1997, doi:10.1016/0191-8141(91)90027-G. gent extension in orogenic belts: The Menderes Mas- Back arc extension and denudation of Mediterranean Buttner, D., and Kowalczyk, G., 1978, Late Cenozoic Stratigra- sif (southwestern Turkey): Geology, v. 23, p. 455–458, eclogites: Tectonics, v. 16, p. 924–941, doi:10.1029/ phy and Paleogeography of Greece—A review, in Closs, doi:10.1130/0091-7613(1995)023<0455:BEIOBT>2.3.CO;2. 97TC02003. H., Roeder, D., and Schmidt, K., eds., Alps, Appenines, Huet, B., Labrousse, L., and Jolivet, L., 2009, Thrust or detach- Avigad, D., Alon, Z., and Garfunkel, Z., 2001, Ductile and brittle Hellenides, v. 38: Stuttgart Schweizerbart, p. 494–501. ment? Exhumation processes in the Aegean: Insight shortening, extension-parallel folds and maintenance Chéry, J., Lucazeau, F., Daignières, M., and Vilotte, J.P., 1992, from a fi eld study on Ios (Cyclades, Greece): Tectonics, of crustal thickness in the central Aegean (Cyclades, Large uplift of rift fl anks: A genetic link with lithospheric v. 28, doi:10.1029/2008TC002397. Greece): Tectonics, v. 20, p. 277–287, doi:10.1029/ rigidity?: Earth and Planetary Science Letters, v. 112, Huet, B., Le Pourhiet, L., Labrousse, L., Burov, E., and Jolivet, 2000TC001190. p. 195–211, doi:10.1016/0012-821X(92)90016-O. L., 2011, Post-orogenic extension and metamorphic Bassi, G., Keen, C.E., and Potter, P., 1993, Contrasting styles Collettini, C., 2011, The mechanical paradox of low-angle core complexes in a heterogeneous crust: The role of of rifting: Models and examples from the eastern Cana- normal faults: Current understanding and open ques- crustal layering inherited from collision. Application to dian margin: Tectonics, v. 12, p. 639–655, doi:10.1029/ tions: Tectonophysics, v. 510, no. 3–4, p. 253–268, the Cyclades (Aegean domain): Geophysical Journal 93TC00197. doi:10.1016/j.tecto.2011.07.015. International, v. 184, p. 611–625, doi:10.1111/j.1365-246X Baziotis, I., and Mposkos, E., 2011, Origin of metabasites Cowie, P.A., and Scholz, C.H., 1992, Displacement-length scal- .2010.04849.x. from Upper Tectonic unit of the Lavrion area (SE Attica, ing relationship for faults: Data synthesis and discus- Huismans, R.S., and Beaumont, C., 2002, Asymmetric litho- Greece): Geochemical implications for dual origin with sion: Journal of Structural Geology, v. 14, p. 1149–1156, spheric extension: The role of frictional plastic strain distinct provenance of blueschist and greenschist’s doi:10.1016/0191-8141(92)90066-6. softening inferred from numerical experiments: Geol- protoliths: Lithos, v. 126, p. 161–173, doi:10.1016/j Davis, E.N., 1982, Geological Map of Greece: Kea Island: Ath- ogy, v. 30, p. 211–214, doi:10.1130/0091-7613(2002)030 .lithos.2011.07.014. ens, Institute of Geology and Mining Research (IGMR), <0211:ALETRO>2.0.CO;2. Baziotis, I., Proyer, A., and Mposkos, E., 2009, High-pressure/ scale 1:50,000. Iglseder, C., Grasemann, B., Schneider, D.A., Petrakakis, K., low-temperature metamorphism of basalts in Lavrion Davis, G.H., 1983, Shear zone model for the origin of meta- Miller, C., Klötzli, U.S., Thöni, M., Zámolyi, A., and Ram- (Greece): Implications for the preservation of peak morphic core complexes: Geology, v. 11, p. 342–347, bousek, C., 2009, I- and S-type plutonism on Serifos metamorphic assemblages in blueschists and green- doi:10.1130/0091-7613(1983)11<342:SMFTOO>2.0.CO;2. (W-Cyclades, Greece): Tectonophysics, v. 473, p. 69–83, schists: European Journal of Mineralogy, v. 21, p. 133– Dawers, N.H., Anders, M.H., and Scholz, C.H., 1993, Growth doi:10.1016/j.tecto.2008.09.021. 148, doi:10.1127/0935-1221/2008/0020-1853. of normal faults: Displacement-length scaling: Geol- Iglseder, C., Grasemann, B., Rice, A.H.N., Petrakakis, K., Bertotti, G., Podladchikov, Y.Y., and Daehler, A., 2000, Dynamic ogy, v. 21, p. 1107–1110, doi:10.1130/0091-7613(1993)021 and Schneider, D.A., 2011, Miocene south directed link between the level of ductile crustal fl ow and style of <1107:GONFDL>2.3.CO;2. low-angle normal fault evolution on Kea Island (West normal faulting of brittle crust: Tectonophysics, v. 320, De Smeth, J.B., 1975, Geological Map of Greece: Kyth- Cycladic Detachment System, Greece): Tectonics, v. 30, p. 195–218, doi:10.1016/S0040-1951(00)00045-7. nos Island: Athens, Institute of Geology and Mining doi:10.1029/2010tc002802. Blake, M.C., Bonneau, M., Geyssant, J., Kienast, J.R., Lepvrier, C., Maluski, H., and Papanikolaou, D., 1981, A geological Research (IGMR), mapped during the years 1972–1973, Jacobshagen, V., 1986, Geologie von Griechenland: Berlin, reconnaissance of the Cycladic blueschist belt, Greece: scale 1:50,000. West Germany, Gebrueder Borntraeger, 363 p. Geological Society of America Bulletin, v. 92, p. 247– Dürr, S., Altherr, R., Keller, J., Okrusch, M., and Seidel, E., Jolivet, L., and Brun, J.-P., 2010, Cenozoic geodynamic evolu- 254, doi:10.1130/0016-7606(1981)92<247:AGROTC>2.0 1978, The median Aegean crystalline belt: Stratigraphy, tion of the Aegean: International Journal of Earth Sci- .CO;2. structure, metamorphism, magmatism, in Cloos , H., ences, v. 99, p. 109–138, doi:10.1007/s00531-008-0366-4. Bonneau, M., 1982, Evolution géodynamique de l’arc égéen Roeder, D., and Schmidt, K., eds., Alps, Apennines, Hel- Jolivet, L., and Patriat, M., 1999, Ductile extension and the depuis le Jurassique Supérieur jusqu’au Miocène: lenides, v. 38: Stuttgart, Schweizerbart, p. 455–476. formation of the Aegean Sea, in Durand, B., Jolivet, L., Bulletin de la Société Géologique de France, v. 7, Faure, M., Bonneau, M., and Pons, J., 1991, Ductile deforma- Horváth, F., and Séranne, M., eds., Tertiary Extension p. 229–242. tion and syntectonic granite emplacement during the within the Alpine Orogen: The Geological Society of Bonneau, M., 1984, Correlation of the Hellenide nappes in the late Miocene extension of the Aegean (Greece): Bulletin London Special Publication 156, p. 427–456. south-east Aegean and their tectonic reconstruction, de la Société Géologique de France, v. 5, p. 3–11. Jolivet, L., Lecomte, E., Huet, B., Denèle, Y., Lacombe, O., in Dixon, J.E., and Robertson, A.H.F., eds., The Geo- Forster, M., and Lister, G., 2009, Core-complex–related exten- Labrousse, L., Le Pourhiet, L., and Mehl, C., 2010, The logical Evolution of the Eastern Mediterranean, v. 17: sion of the Aegean lithosphere initiated at the Eocene- North Cycladic detachment system: Earth and Plan- The Geological Society, London, Special Publications, Oligocene transition: Journal of Geophysical Research, etary Science Letters, v. 289, p. 87–104, doi:10.1016/j p. 517–527. v. 114, p. doi:10.1029/2007JB005382. .epsl.2009.10.032. Brichau, S., Ring, U., Ketcham, R., Carter, A., Stockli, D., and Fytikas, M., 1977, Geological Map of Greece: Milos Island: Ath- Keay, S., Lister, G., and Buick, I., 2001, The timing of par- Brunel, M., 2006, Constraining the long-term evolution ens, Institute of Geology and Mining Research (IGMR), tial melting, Barrovian metamorphism and granite of the slip rate for a major extensional fault system in mapped during the years 1971–1973, scale 1:25,000. intrusion in the Naxos metamorphic core complex, the central Aegean, Greece, using thermochronology: Gautier, P., and Brun, J.-P., 1994, Crustal-scale geometry and Cyclades, Aegean Sea, Greece: Tectonophysics, v. 342, Earth and Planetary Science Letters, v. 241, p. 293, kinematics of late-orogenic extension in the central p. 275–312, doi:10.1016/S0040-1951(01)00168-8. doi:10.1016/j.epsl.2005.09.065. Aegean (Cyclades and Ewia Island): Tectonophysics, Keiter, M., Tomaschek, F., and Ballhaus, C., 2008, The struc- Brichau, S., Ring, U., Carter, A., Bolhar, R., Monie, P., Stockli, v. 238, p. 399–424, doi:10.1016/0040-1951(94)90066-3. tural evolution of Kythnos Island (Cyclades, Greece)— D., and Brunel, M., 2008, Timing, slip rate, displacement Gautier, P., Brun, J.-P., and Jolivet, L., 1993, Structure and A reconnaissance: Zeitschrift der Deutschen Gesell- and cooling history of the Mykonos detachment foot- kinematics of Upper Cenozoic extensional detachment schaft für Geowissenschaften, v. 159, p. 513–520, wall, Cyclades, Greece, and implications for the open- on Naxos and Paros (Cyclades Islands, Greece): Tecton- doi:10.1127/1860-1804/2008/0159-0513. ing of the Aegean Sea basin: Journal of the Geological ics, v. 12, p. 1180–1194, doi:10.1029/93TC01131. Krohe, A., Mposkos, E., Diamantopoulos, A., and Kaouras, Society of London, v. 165, p. 263–277, doi:10.1144/0016 Gessner, K., Ring, U., Johnson, C., Hetzel, R., Passchier, C., G., 2010, Formation of basins and mountain ranges in -76492006-145. and Güngör, T., 2001, An active bivergent rolling-hinge Attica (Greece): The role of Miocene to Recent low-angle Brichau, S., Thomson, S., and Ring, U., 2010, Thermochro- detachment system: Central Menderes metamorphic normal detachment faults: Earth-Science Reviews, nometric constraints on the tectonic evolution of the core complex in western Turkey: Geology, v. 29, p. 611– v. 98, p. 81–104, doi:10.1016/j.earscirev.2009.10.005.

38 www.gsapubs.org | Volume 4 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021 Miocene bivergent extension in the Aegean | RESEARCH

Kuhlemann, J., Frisch, W., Dunkl, I., Kázmér, M., and and Boger, H., 1982, Remnants of a Late Cretaceous Architecture, Rheology and Heat Budget: The Geological Schmiedl, G., 2004, Miocene siliciclastic deposits of high temperature belt on the island of Anafi (Cyclades, Society, London, Special Publication 321, p. 139–167. Naxos Island: Geodynamic and environmental impli- Greece): Neues Jahrbuch für Mineralogie-Abhandlun- Tirel, C., Brun, J.-P., and Burov, E., 2008, Dynamics and struc- cations for the evolution of the southern Aegean Sea gen, v. 145, p. 157–182. tural development of metamorphic core complexes: (Greece), in Bernet, M., and Spiegel, C., eds., Detrital Ring, U., and Layer, P.W., 2003, High-pressure metamorphism Journal of Geophysical Research, v. 113, doi:10.1029/ Thermochronology—Provenance Analysis, Exhuma- in the Aegean, eastern Mediterranean: Underplating and 2005JB003694. tion, and Landscape Evolution of Mountain Belts: Geo- exhumation from the Late Cretaceous until the Miocene Tirel, C., Gautier, P., van Hinsbergen, D.J.J., and Wortel, M.J.R., logical Society of America Special Paper 378, p. 51–65. to Recent above the retreating Hellenic subduction zone: 2009, Sequential development of interfering metamor- Lecomte, E., Jolivet, L., Lacombe, O., Denèle, Y., Labrousse, Tectonics, v. 22, 1022, doi:1010.1029/2001TC001350. phic core complexes: Numerical experiments and com- L., and Le Pourhiet, L., 2010, Geometry and kinemat- Ring, U., and Reischmann, T., 2002, The weak and superfast parison with the Cyclades, Greece, in Van Hinsbergen, ics of Mykonos detachment, Cyclades, Greece: Evi- Cretan detachment, Greece: Exhumation at subduction D.J.J., Edwards, M.A., and Govers, R., eds., Collision and dence for slip at shallow dip: Tectonics, v. 29, TC5012, rates in extruding wedges: Journal of the Geological Collapse at the Africa-Arabia-Eurasia Subduction Zone: doi:10.1029/2009TC002564. Society of London, v. 159, p. 225–228, doi:10.1144/0016 The Geological Society, London, Special Publication 311, Lee, J., and Lister, G.S., 1992, Late Miocene ductile exten- -764901-150. p. 257–292. sion and detachment faulting, Mykonos, Greece: Geol- Ring, U., Layer, P.W., and Reischmann, T., 2001, Miocene high- Trotet, F., Jolivet, L., and Vidal, O., 2001, Tectono-metamor- ogy, v. 20, p. 121–124, doi:10.1130/0091-7613(1992)020 pressure metamorphism in the Cyclades and , phic evolution of Syros and Sifnos Islands (Cyclades, <0121:LMDEAD>2.3.CO;2. Aegean Sea, Greece: Evidence for large-magnitude dis- Greece): Tectonophysics, v. 338, p. 179–206, doi:10.1016/ Le Pichon, X., Angelier, J., Osmaston, M.F., and Stegena, placement on the Cretan detachment: Geology, v. 29, S0040-1951(01)00138-X. L., 1981, The Aegean Sea: Philosophical Transactions no. 5, p. 395–398, doi:10.1130/0091-7613(2001)029<0395: Tschegg, C., and Grasemann, B., 2009, Deformation and of the Royal Society of London, ser. A, Mathematical MHPMIT>2.0.CO;2. alteration of a granodiorite during low-angle normal and Physical Sciences, v. 300, p. 357–372, doi:10.1098/ Ring, U., Will, T., Glodny, J., Kumerics, C., Gessner, K., Thom- faulting (Serifos, Greece): Lithosphere, v. 1, p. 139–154, rsta.1981.0069. son, S., Güngör, T., Monié, P., Okrusch, M., and Drüp- doi:10.1130/L33.1. Liati, A., Skarpelis, N., and Pe-Piper, G., 2009, Late Miocene pel, K., 2007, Early exhumation of high-pressure rocks Tullis, J., and Yund, R.A., 1987, Transition from cataclastic fl ow magmatic activity in the Attic-Cycladic belt of the in extrusion wedges: Cycladic Blueschist unit in the to dislocation creep of feldspar: Mechanisms and micro- Aegean (Lavrion, SE Attica, Greece): Implications for eastern Aegean, Greece, and Turkey: Tectonics, v. 26, structures: Geology, v. 15, p. 606–609, doi:10.1130/0091 the geodynamic evolution and timing of ore deposition: TC2002, doi:10.1029/2005TC001872. -7613(1987)15<606:TFCFTD>2.0.CO;2. Geological Magazine, v. 146, p. 732–742, doi:10.1017/ Ring, U., Glodny, J., Will, T., and Thomson, S., 2010, The Hel- Urai, J.L., Schuiling, R.D., and Jansen, J.B.H., 1990, Alpine S0016756809006438. lenic subduction system: High-pressure metamorphism, deformation on Naxos (Greece), in Knipe, R.J., and Rut- Lister, G.S., and Davis, G.A., 1989, The origin of metamorphic exhumation, normal faulting, and large-scale extension: ter, E.H., eds., Deformation Mechanisms, Rheology and core complexes and detachment faults formed during Annual Review of Earth and Planetary Sciences, v. 38, Tectonics: The Geological Society of London Special Tertiary continental extension in the northern Colorado p. 45–76, doi:10.1146/annurev.earth.050708.170910. Publication 54, p. 509–522. River region, U.S.: Journal of Structural Geology, v. 11, Ring, U., Glodny, J., Will, T., and Thomson, S., 2011, Normal Vandenberg, L.C., and Lister, G.S., 1996, Structural analysis of p. 65–94, doi:10.1016/0191-8141(89)90036-9. faulting on Sifnos and the South Cycladic detachment basement tectonites from the Aegean metamorphic core Lister, G.S., Banga, G., and Feenstra, A., 1984, Metamorphic system, Aegean Sea, Greece: Journal of the Geological complex of Ios, Cyclades, Greece: Journal of Structural core complexes of Cordilleran type in the Cyclades, Society of London, v. 168, p. 751–768, doi:10.1144/0016 Geology, v. 18, p. 1437–1454, doi:10.1016/S0191-8141(96) Aegean Sea, Greece: Geology, v. 12, p. 221–225, -76492010-064. 00068-5. doi:10.1130/0091-7613(1984)12<221:MCCOCT>2.0.CO;2. Salemink, J., and Schuiling, R.D., 1987, A two-stage, transient Vanderhaeghe, O., 2004, Structural development of the Naxos Lister, G.S., Etheridge, M.A., and Symonds, P.A., 1986, Detach- heat and mass transfer model for the granodiorite intru- migmatite dome, in Whitney, D.L., Teyssier, C., and Sid- ment faulting and the evolution of passive continental sion at Seriphos, Greece, and the associated formation doway, C.S., eds., Gneiss Domes in Orogeny: Geological margins: Geology, v. 14, p. 246–250, doi:10.1130/0091 of contact metasomatic skarn and Fe-ore deposits, in Society of America Special Paper 380, p. 211–227. -7613(1986)14<246:DFATEO>2.0.CO;2. Helgeson, H.C., ed., Chemical transport in metasomatic van der Maar, P.A., and Jansen, J.B.H., 1983, The geology of Marinos, G., 1951, Geology and metallogenesis of Seri- processes—NATO ASI Series. Series C: Mathematical the polymetamorphic complex of Ios, Cyclades, Greece, phos Island, Volume I: Athens, Geological Geophysical and Physical Sciences, v. 218: Dordrecht, Kluwer Aca- and its signifi cance of the Cycladic Massif: Geologische Research, p. 95–127. demic Publishers, p. 547–575. Rundschau, v. 72, p. 283–299, doi:10.1007/BF01765910. Mascle, J., and Martin, L., 1990, Shallow structure and recent Sánchez-Gómez, M., Avigad, D., and Heimann, A., 2002, van Hinsbergen, D.J.J., Zachariasse, W.J., Wortel, M.J.R., evolution of the Aegean Sea: A synthesis based on Geochronology of clasts in allochthonous Miocene and Meulenkamp, J.E., 2005, Underthrusting and continuous refl ection profi les: Marine Geology, v. 94, sedimentary sequences on Mykonos and Paros Islands: exhumation: A comparison between the External Hel- p. 271–299, doi:10.1016/0025-3227(90)90060-W. Implications for back-arc extension in the Aegean lenides and the hot Cycladic and cold McKenzie, D.P., 1978, Some remarks on the development of Sea: Journal of the Geological Society of India, v. 159, core complexes (Greece): Tectonics, v. 24, TC2011, sedimentary basins: Earth and Planetary Science Let- p. 45–60, doi:10.1144/0016-764901031. doi:10.1029/2004TC001692. ters, v. 40, p. 25–32, doi:10.1016/0012-821X(78)90071-7. Schliestedt, M., Bartsch, V., Carl, M., Matthews, A., and Walsh, J.J., and Watterson, J., 1988, Analysis of the rela- Mehl, C., Jolivet, L., and Lacombe, O., 2005, From ductile to Henjes-Kunst, F., 1994, The P-T path of greenschist-facies tionship between displacements and dimensions of brittle: Evolution and localization of deformation below rocks from the island of Kithnos (Cyclades, Greece): faults: Journal of Structural Geology, v. 10, p. 239–247, a crustal detachment (Tinos, Cyclades, Greece): Tecton- Chemie der Erde, v. 54, p. 281–296. doi:10.1016/0191-8141(88)90057-0. ics, v. 24, no. TC4017, doi:10.1029/2004TC001767. Schneider, D., Senkowski, C., Vogel, H., Grasemann, B., and Igl- Weil, J., Petrakakis, K., Grasemann, B., and Iglseder, C., Mehl, C., Jolivet, L., Lacombe, O., Labrousse, L., and Rim- seder, C., 2011, Eocene tectonometamorphism on Serifos 2010, Blueschists and eclogites from southern Sifnos mele, G., 2007, Structural evolution of Andros (Cyclades, (western Cyclades) deduced from zircon depth-profi ling (Cyclades, Greece), in Proceedings European Geo- Greece): A key to the behaviour of a (fl at) detachment geochronology and mica thermochronology: Lithos, physical Union General Assembly 2010, Vienna, 2010: with an extending continental crust, in Taymaz, T., Yilmaz, v. 125, p. 151–172, doi:10.1016/j.lithos.2011.02.005. Geophysical Research Abstracts, v. 12, EGU2010-14114. Y., and Dilek, Y., eds., The Geodynamics of the Aegean Shaked, Y., Avigad, D., and Garfunkel, Z., 2000, Alpine high- Wernicke, B., 1985, Uniform-sense normal simple shear of and Anatolia: The Geological Society of London Special pressure metamorphism at the Almyropotamos win- the continental lithosphere: Canadian Journal of Earth Publication 291, p. 41–73. dow (southern Evia, Greece): Geological Magazine, Sciences, v. 22, p. 108–125, doi:10.1139/e85-009. Morales, L.F.G., Casey, M., Lloyd, G.E., and Williams, D.M., v. 137, p. 367–380, doi:10.1017/S001675680000426X. Wijbrans, J., and McDougall, I., 1988, Metamorphic evolu- 2011, Kinematic and temporal relationships between Skarpelis, N., Tsikouras, B., and Pe-Piper, G., 2008, The Mio- tion of the Attic Cycladic metamorphic belt on Naxos parallel fold hinge lines and stretching lineations: A cene igneous rocks in the Basal unit of Lavrion (SE (Cyclades, Greece) utilizing 40Ar/ 39Ar age spectrum microstructural and crystallographic preferred orien- Attica, Greece): Petrology and geodynamic implica- measurements: Journal of Metamorphic Geology, v. 6, tation approach: Tectonophysics, v. 503, p. 207–221, tions: Geological Magazine, v. 145, p. 1–15, doi:10.1017/ p. 571–594, doi:10.1111/j.1525-1314.1988.tb00441.x. doi:10.1016/j.tecto.2011.03.003. S0016756807003949. Wijbrans, J., Schliestedt, M., and York, D., 1990, Single grain Niemeijer, A.R., and Spiers, C.J., 2007, A microphysical model Stipp, M., Stunitz, H., Heilbronner, R., and Schmid, S.M., 2002, argon laser probe dating of phengites from the blue- for strong velocity weakening in phyllosilicate-bearing The eastern Tonale fault zone: A “natural laboratory” for schist to greenschist transition on Sifnos (Cyclades, fault gouges: Journal of Geophysical Research, v. 112, crystal plastic deformation of quartz over a temperature Greece): Contributions to Mineralogy and Petrology, doi:10.1029/2007JB005008. range from 250 to 700°C: Journal of Structural Geology, v. 104, p. 582–593, doi:10.1007/BF00306666. Passchier, C.W., and Trouw, R.A.J., 2005, Microtectonics: Ber- v. 24, p. 1861–1884, doi:10.1016/S0191-8141(02)00035-4. Zeffren, S., Avigad, D., Heimann, A., and Gvirtzman, Z., 2005, lin, Springer-Verlag, 366 p. Stouraiti, C., Mitropoulos, P., Tarney, J., Barreiro, B., McGrath, Age resetting of hanging wall rocks above a low-angle Pe-Piper, G., and Piper, D.J.W., 2002, The Igneous Rocks of A.M., and Baltatzis, E., 2010, Geochemistry and petro- detachment fault: Tinos Island (Aegean Sea): Tectono- Greece: Berlin, Borntraeger, 573 p. genesis of late Miocene granitoids, Cyclades, southern physics, v. 400, p. 1–25, doi:10.1016/j.tecto.2005.01.003. Regenauer-Lieb, K., Rosenbaum, G., and Weinberg, R.F., 2008, Aegean: Nature of source components: Lithos, v. 114, Strain localisation and weakening of the lithosphere p. 337–352, doi:10.1016/j.lithos.2009.09.010. MANUSCRIPT RECEIVED 3 JULY 2011 during extension: Tectonophysics, v. 458, p. 96–104, Thomson, S.N., Ring, U., Brichau, S., Glodny, J., and Will, T.M., REVISED MANUSCRIPT RECEIVED 24 OCTOBER 2011 doi:10.1016/j.tecto.2008.02.014. 2009, Timing and nature of formation of the Ios meta- MANUSCRIPT ACCEPTED 26 OCTOBER 2011 Reinecke, T., Altherr, R., Hartung, B., Hatzipanagiotou, K., morphic core complex, southern Cyclades, Greece, in Kreuzer, H., Harre, W., Klein, H., Keller, J., Geenen, E., Ring, U., and Wernicke, B., eds., Extending a Continent: Printed in the USA

LITHOSPHERE | Volume 4 | Number 1 | www.gsapubs.org 39

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/1/23/3038512/23.pdf by guest on 29 September 2021