The Late Cretaceous Asteroussia event as recorded in the Cyclades is a
potential key to the tectonic evolution of Western Tethys. Microstructural
analysis and 40Ar/39Ar geochronology on garnet–mica schists and
the underlying granitoid basement terrane on the island of Ios demonstrates
evidence of a Late Cretaceous high-pressure, medium-temperature (HP–MT)
metamorphic event. This suggests that the Asteroussia crystalline nappe on
Crete extended northward to include these Gondwanan tectonic slices. In this
case, the northern part of the Asteroussia nappe (on Ios) is overlain by the
terrane stack defined by the individual slices of the Cycladic
Eclogite–Blueschist Unit, whereas in the south (in Crete) the Asteroussia
slices are near the top of a nappe stack defined by the individual tectonic
units of the external Hellenides. This geometry implies that accretion of the
Ios basement terrane involved a significant leap of the subduction megathrust
(250–300 km) southward. Accretion needs to have commenced at or
about ∼38Ma, when the already partially exhumed slices of the
Cycladic Eclogite–Blueschist Unit began to thrust over the Ios basement. By
∼35–34 Ma, the subduction jump had been accomplished, and
renewed rollback began the extreme extension that led to the exhumation of the
Ios metamorphic core complex.
Introduction
A terrane stack accreted on the northern edge of the Tethys Ocean during the
episodic closure of this ocean basin. Several of these tectonic slices now
outcrop on the island of Ios, in the Cyclades, Greece (e.g., Dürr et al.,
1978; Andriessen et al., 1987; Forster and Lister, 1999a, b; Ring et al.,
2007; Forster and Lister, 2009). Tectonic slices in the Cycladic Blueschist
Unit were subject to high-pressure metamorphism and later juxtaposed against
tectonic slices of Hercynian continental basement. How this juxtaposition
occurred remains controversial (Ring et al., 2007; Huet et al., 2009; Forster
and Lister, 2009, and references therein). One of the competing hypotheses is
that a succession of tectonic mode switches took place, with episodes of
crustal shortening prior to each of a succession of accretion events. Each
accretion event appears to have been followed by an episode of crustal
extension (Forster and Lister 2009). Extension following the accretion events
that occurred later in this history caused core complex formation, including
the formation of the first recognised Aegean metamorphic core complexes
(Lister et al., 1984). The hypothesis that is the main contender as an
opposing point of view is that this did not occur, and the Cycladic Blueschist
Unit continually extruded during the long history of Alpine convergence (in
the so-called orogenic phase, Huet et al., 2009). However, Forster and Lister
(2009) unequivocally demonstrate that a discrete succession of exhumation
events juxtaposed the Hercynian granitoid basement (identified as the Ios
basement terrane in this paper) against the overlying Cycladic
eclogite–blueschist slices, so this hypothesis (at least in its present form)
is not tenable. Either the Cycladic Blueschist Unit must have been earlier
overthrust (at ∼38Ma, Forster and Lister, 2009) and largely
eroded, or, alternatively, at about ∼38Ma, the Ios basement
terranes must have begun to subduct beneath an already largely exhumed
Cycladic Blueschist Unit. In this case, subduction must have continued until
these Gondwanan terranes were accreted at ∼35Ma. Their
subsequent crustal extension involved a succession of extensional ductile
shear zones and later-formed detachment faults.
Dispute arises in part because insufficient information is available as to the
details of the timing and thermal evolution of individual rock units, in
particular those in the Ios basement terranes that are the focus of this
paper. In the extrusion model the Ios basement terranes are overridden as the
result of thrust-induced extrusion, with deep crustal materials extruded above
thrust faults that operated under continuous plate convergence, with little to
no horizontal stretching (Ring et al., 2007; Huet et al., 2009). In the
tectonic mode switching model, a tectonic shuffle zone must have been created
in the upper levels of the Ios basement terranes, and this must have been
later truncated during detachment faulting. Multiple shuffling events are
implied by the several switches between horizontal shortening and horizontal
stretching triggered by roll back (Lister et al., 2001; Forster and Lister,
2009; Forster et al., 2020). The marked contrast in the detail required by
these competing hypotheses makes it evident that a significant knowledge gap
exists in understanding the succession of discrete deformation and
metamorphism events in the upper structural levels of the Ios basement
terranes, in particular those that occurred prior to Alpine deformation and
separately those that occurred prior to exhumation of the Ios basement terrane
(Forster and Lister, 2009; Lister and Forster 2016; Yeung, 2019; Forster
et al., 2020). This research project was undertaken in order to begin to
remedy this deficiency.
Early researchers in the area assumed that, prior to Alpine time, the Ios basement was
affected only by Hercynian deformation and metamorphism, based on age data
from hornblende and zircon (Andriessen et al., 1987). However, white mica
deformation fabrics in the Ios augengneiss core consistently yield
40Ar/39Ar ages of ∼70–80 Ma (Forster and Lister,
2009). This led us to investigate the possibility that the Ios basement
terrane that was made up of garnet–mica schist and augengneiss could be part
of the Asteroussia nappe (see Be'eri-Shlevin et al., 2009). Previous
interpretation of such age data (e.g., Andriessen et al., 1987; Baldwin and
Lister, 1998) considered only the effects of “excess argon” or “mixing”
and suggested that the apparent Late Cretaceous ages were the result of the
Hercynian (∼300Ma) argon population mixing with Cenozoic (∼50Ma or younger) gas population. However, if this was the case,
precisely defined frequently measured ages (FMAs) would not exist in age
probability plots. Therefore, we were led to consider that the
70–80 Ma date reported in the structurally deepest augengneiss of the
Ios lower plate was in fact the characteristic age of the “Asteroussia
event”.
To progress, we need to demonstrate that the effects of such an event can be
distinguished in the complex history of deformation and metamorphism (and
fluid alteration) experienced by these rocks. Therefore, we re-examined
outcrops in the northwest corner of the basement terranes on Ios, in an
attempt to determine the significance of the previously reported
70–80 Ma ages. We combined a field study with microstructural
analysis and 40Ar/39Ar geochronology to address (i) the
character and location of micro-deformation structures with late Cretaceous
age and (ii) the time relations between various metamorphic and deformation
events. Our study identified relicts of earlier fabrics in low-strain zones
that “survived” later shear zone operation. 40Ar/39Ar
geochronology on these fabrics demonstrate Late Cretaceous high-pressure
metamorphism, specifically with the growth of phengitic mica in the
augengneiss terrane and the overlying garnet–mica schist. The high retentivity
of argon in phengitic white mica (Forster and Lister, 2014) allowed these ages
to survive the thermal effects of the later Alpine history.
The Asteroussia Nappe
Late Cretaceous-aged metamorphic events were first reported from small klippens
outcropped near the Asteroussia mountains in Crete, and later in various
Cycladic islands (e.g., Be'eri-Shlevin et al., 2009; Dürr et al., 1978;
Seidel et al., 1976). This unit is identified as the Asteroussia nappe,
positioned near or at the top of the Aegean terrane stack (except where
klippen of Cycladic blueschist occurred above the unit), and reflects the
imprint of metamorphism in the time range 70–80 Ma (Bonneau, 1972, 1984). The newly defined terrane was to be characterised as having
Cretaceous high-temperature, low-pressure (HT-LP) metamorphic assemblages
associated with granitoid intrusions with peak metamorphic conditions in the
Late Cretaceous, at ∼70Ma (Dürr et al., 1978; Langosch et
al., 2000; Patzak et al., 1994; Seidel et al., 1976). Table 3 shows data from
other researchers who then reported Late Cretaceous ages in outcrops occurring
as small klippen on various Cycladic islands (Tinos, Andros, Syros, Donoussa,
Ikaria, Nikouria and Anafi) as well as in further outcrops on Crete (Avigad
and Garfunkel, 1989; Be'eri-Shlevin et al., 2009; Bröcker and Franz, 2006;
Dürr et al., 1978; Langosch et al., 2000; Patzak et al., 1994; Seidel
et al., 1976). In turn this led Be'eri-Shlevin et al. (2009) to note that
although published Rb-Sr dates on amphiboles from the Asteroussia nappe range
from ∼45–85 Ma, the dates cluster at ∼70Ma. These authors therefore extended the areal extent of the
Asteroussia nappe to cover a north–south distance of ∼300km
(Fig. 1).
Map of the Cyclades and Crete, dotted line illustrates published
extent of the Asteroussia nappe: area enclosed with the dashed line includes
outcrop localities with late Cretaceous age (with information retrieved from
Be'eri-Shlevin et al., 2009). The area shaded in light blue is the revised
areal extent of the Asteroussia nappe suggested in this paper.
The geometry of the Asteroussia nappe is complex, however, with notable local
variations when comparing examples from Crete with examples in the northern
Aegean Sea. For instance, on Crete a Late Cretaceous event was reported in
metapelites which correlates in age with the complex upper unit of the terrane
stack in some areas of the Cyclades (Avigad and Garfunkel, 1989;
Be'eri-Shlevin et al., 2009; Bröcker and Franz, 1998, 2006; Pe-Piper and Photiades, 2006). However, the Asteroussia outcrops
in Crete are small tectonic klippen (up to 10–15 km wide) with poor
lithological and structural correlations (Dürr et al., 1978; Seidel
et al., 1976, 1981). In contrast, in the Cyclades, island-scale
structural models involve two to four tectonic slices, and there are
reasonable correlations that can be made across the entire archipelago.
Nevertheless, some examples in the Cyclades involve meta-ophiolites and
mélange zones that first underwent blueschist facies and then were
overprinted by retrograde greenschist facies metamorphism. In other cases,
Asteroussia klippen overlie high-pressure metamorphic rocks from the Cycladic
Blueschist Unit (e.g., Avigad and Garfunkel, 1989; Be'eri-Shlevin et al.,
2009; Bröcker and Franz, 1998, 2006; Pe-Piper and
Photiades, 2006; Ring et al., 2003). Ios stands out in that the Asteroussia
ages have been obtained from the lowermost structural slices in the terrane
stack, with clear proof that these are Gondwanan in their affinity (Keay and
Lister, 2002). Hence, although we agree with Be'eri-Shlevin et al. (2009) that
the different Cycladic outcrops in the north (Andros, Tinos, Syros, Ikaria)
and in the south (Ikaria, Donoussa, Nikouria, Anafi) are part of an extensive
Asteroussia nappe that once extended northward from Crete, this observation
requires the Asteroussia nappe in its entirety to have been Gondwanan in its
origin. Tectonic shuffling involving large horizontal relative motions is well
capable of explaining the observed complexity, but only if the terrane was
first accreted in its entirety while subject to out-of-sequence thrusting and
then affected by extreme crustal extension.
Correlations between the results of previous work and the tectonic
sequence diagrams (TSDs) developed in this paper. The results imply that
parts of the garnet–mica schist have been shared some parts of history of
high-pressure/low-temperature metamorphism as occurred in the lowermost
schist units of the Cycladic Blueschist Unit.
The Asteroussia event on Ios
The terrane stack outcropping on Ios involves thin tectonic slices separated
by island-scale deformation structures such as detachment faults and/ or
ductile shear zones (e.g., Forster and Lister, 2009; Forster et al., 2020).
All terranes experienced Early Oligocene stretching (Fig. 2) and were
variably affected by the south-directed South Cyclades Shear Zone (SCSZ,
D2 in other publications) (e.g., Forster and Lister 1999a, b, 2009; Forster et al., 2020; Huet et al., 2009, 2011; Ring
et al., 2007). An Eocene–Oligocene high-pressure terrane (the Cycladic
blueschist unit, upper plate of the Ios Detachment) overlies the Gondwanan Ios
basement terrane. There are three tectonic slices in the basement, each
recording slightly different metamorphic histories (refer to maps in
Supplement). The top-most Port Beach tectonic slice contains two lithologies
(garnet–mica schist above augengneiss) and lies immediately beneath the Ios
detachment (Forster and Lister 1999a). Beneath this tectonic slice is the
∼500m thick garnet–mica schist unit (of variable thickness
across the island) at mid-structural level and the structurally deepest
augengneiss core of the Ios metamorphic core complex (e.g., Andriessen et al.,
1987; Baldwin and Lister, 1998; Forster and Lister, 2009; Vandenberg and
Lister, 1996).
Schematic cross sections illustrating the slab-peel model. The
orange star illustrates the relative movement of subducting material across
time: (a) the Gondwanan Asteroussia terrane arrives at the subduction zone;
(b) as it accretes to the terrane stack, the Asteroussia terrane underplates
the youngest slice of the terrane stack; (c) the subduction megathrust jumps
∼300km southward, slicing the Asteroussia terrane from the
subducting African slab, while subduction goes on and the slab continues to
peel away from the “buoyant” Asteroussia terrane. Asthenosphere penetrates
to the dilating megathrust (d) while the subducting African slab continues
to roll back and the break widens. Melting of the uplifting asthenosphere
causes extensive magmatism.
The slab-necking–drop-off model proposed in this paper: (a) the
Asteroussia terrane arrives at the Aegean terrane stack; (b) the subduction
zone jams, so the megathrust leaps southward while the African slab begins
to neck and roll back; (c) by ∼32Ma the subduction jump is
accomplished, breaking the stretching slab; (d) a new subduction zone
develops in the south, with fluids rising from the slab causing melting.
Rollback of the newly formed slab triggers extreme extension across the
Aegean, exhuming metamorphic rocks and forming the Ios metamorphic core
complex.
Figure 1 shows the areal extent of the Asteroussia terrane based on
Be'eri-Shlevin et al. (2009), with the shaded area indicating the revised
extent according to what we report in this paper. Importantly, this study
recognises the exhumed Asteroussia terrane north of Crete, in the Ios basement
terrane across the terrane stack, which enabled the identification of a
subduction jump that is impossible without a tectonic mode switch. If correct,
this is a significant modification implying a widespread metamorphic event in
northern Tethys during Late Cretaceous time, including most of the Cycladic
islands as reported in this paper and across the Mediterranean region (Altherr
et al., 1994; Be'eri-Shlevin et al., 2009; Langosch et al., 2000). Moreover,
if the extent of the basement terrane is as large as indicated in Fig. 1, its
accretion to the modern terrane stack in the latest Eocene time implies a
southward jump of the subduction zone megathrust exceeding 250–300 km
(Figs. 2 and 3). Potentially these terranes were autochthonous, with their
final accretion involving a period of flat slab subduction followed by the
initiation of a new subduction zone (Fig. 3c–d). Subsequent rollback can then
stretch the Cycladic crust, explaining the variation in crustal thickness from
∼32km thick beneath the Cyclades, to 18 km beneath the
Sea of Crete, and ∼30km beneath Crete (based on Makris and
Vees, 1976 and Makris et al., 2001). Moreover, the subduction jump is able to
explain the formation of core complexes during extreme extension caused by
rollback after accretion, and the later Oligocene–Miocene magmatic event that
is observed across the Cyclades.
Microstructural and mineral chemistry analyses across three tectonic slices in the Ios basement terrane
Seven samples were selected (Table 2). The augengneiss (IO18-01) was collected
from the structurally shallowest Port Beach tectonic slice, which has a
pervasive south-directed shear fabric, with many intensive S-directed shear
bands. Four samples were collected (IO17-03, IO17-05, IO17-04, IO18-05) from
the deformed garnet–mica schist that underplates the Port Beach tectonic
slice, each collected from a different depth of this structurally mid-level
tectonic slice, which preserves different stages of the SCSZ operation. Two samples (AG03-03, AG03-05) were examined from the
upper levels of the structurally lowest unit, the augengneiss core. Results
from these samples were first reported by Forster and Lister (2009) but were
re-examined to establish the association between microstructure and their Late
Cretaceous ∼70–80 Ma ages.
List of samples.
SampleRock type and mineralogySample location Deformation structure analysedAgesLat. (∘ N)Long. (∘ E)Port Beach tectonic slice (structurally highest in the Ios lower plate)IO18-01augengneiss: quartz ± garnet ± potassium feldspar ± hornblende ± white mica ± biotite36∘42.9′ N25∘17.2′ NFirst-generation (pre-shear zoneoperation) white mica as porphyroclasts in matrix K-feldspar crystals in groundmass192±1.3Ma188±1.0Ma84.2±2.3Ma592±8.7Ma166±2.8Ma39.8±3.5MaMylopotas tectonic slice (structurally mid-level in the Ios lower plate)IO17-03aGarnet–mica schist: quartz ± garnet ± biotite ± rutile ±white mica ± potassium feldspar36∘42.9′ N25∘17.2′ EWhite mica from south-directed shear zone deformation fabrics76.9±0.7Ma36.0±0.5MaIO17-04 Garnet–mica schist: quartz ± garnet ± hornblende ± biotite ± potassium feldspar ± white mica36∘42.9′ N25∘17.2′ EWhite mica from south-directed shear zone deformation fabrics163±1.0Ma174±1.1MaIO17-05a,bGarnet–mica schist: quartz ± hornblende ± garnet ± biotite ± potassium feldspar ± white mica36∘42.9′ N25∘17.2′ EWhite mica from south-directed shear zone deformation fabrics81.0±0.6Ma58.8±1.5MaIO18-05bGarnet–mica schist: quartz ± garnet ± biotite ± rutile ± white mica ± potassium feldspar36∘42.5′ N25∘17.2′ EWhite mica from south-directed shear zone deformation fabrics, sample represents the structurally lowest level of this tectonic unit50.7±0.4Ma43.8±1.1MaAugengneiss basement (structurally lowest in the Ios lower plate)AG03-03 Re-analysis on Ar/Argeochronology data producedand published in Forster and Lister (2009).augengneiss:36∘42.2′ N25∘17.2′ EWhite mica from south-directed shear zonefabrics overprinted by north-directed shearzone Groundmass and porphyroclast K-feldspar grains subjected to deformation by two shear zones73.9±0.6Ma70.4±0.8Ma84.8±0.7Ma∼13±0.1MaAG03-05 Quartz ± biotite ± hornblende ± white mica ± potassium feldspar36∘42.2′ N25∘17.3′ EWhite mica from south-directed shear zone fabrics overprinted by north-directed shear zone72±0.6Ma68.3±0.3Ma
Samples stored in collections of the Structure Tectonics
Team, Research School of Earth Sciences, Australian National University,
Canberra, 2601 Australia.a Rock sample preserved evidence of eclogite facies after the
first (deformation) fabric.b The younger age from the less retentive argon diffusion domain in the grain analysed is comparable with the Δ1B eclogite event in Syros.
Previous structural studies in the Ios basement generally did not recognise
the presence of high-pressure rocks in the Ios basement, and its tectonic
history is generally identified as the M0 event (Vandenberg and Lister,
1996; Baldwin and Lister, 1998; Forster and Lister, 1999b, etc.). However, the
history of deformation and metamorphism in these rocks is more complex than
such simple notations imply. Therefore, we applied the method of tectonic
sequence diagrams (TSDs) presented in Forster and Lister (2008) and Forster
et al. (2020) to document the effects of the succession of pre-Alpine to
Oligocene metamorphic episodes that can be observed (Table 1). The sequence
of metamorphic mineral growth and deformation events is consistent from place
to place throughout the entire shear zone carapace of the exhumed Ios
basement, with the order of mineral growth episodes tied to different fabrics
produced during ductile shear zone operation and/or pure shear ductile
stretching of the rock mass. Table 1 compares the results of this analysis
with the traditional D1, D2, … Dn method. The detail of
relative time constraints could be accurately delineated using these TSDs.
TSDs tie metamorphic evolution to the sequence of fabric-forming events and to
the processes that took place during the microstructural evolution and are
therefore critical in enabling the link between the results of
40Ar/39Ar geochronology to the detail of microstructural
observations. We were able to link dates to specific deformation fabric and
mineral growth events and thus demonstrate that some of these relict fabrics
preserved remnant microstructures from earlier pre-Alpine deformation
events. The effects of a high-pressure late Cretaceous event are evident in
these relict fabrics in the Ios basement, so we conclude that the pre-Alpine
tectonic history prior to accretion is more complicated than a single event
would allow.
Port Beach tectonic slice: the structurally shallowest level
The Port Beach tectonic slice just beneath the Ios detachment represents the
structurally shallowest level of the Ios basement terrane and is made up of
two lithological units: a thin slice of structurally above garnet–mica schist
and the underplating augengneiss with quartz porphyroclasts. Both units
preserved numerous recumbently folded veins, isoclinal folds and boudinage
structures overprinted by the south-directed SCSZ. A section of altered,
greenschist facies garnet–mica schist with chloritoid replacing garnets was
observed in the garnet–mica schist tectonic slice near the tectonic contact
between Ios upper and lower plate. Beneath the altered zone, garnet
porphyroblasts in the garnet–mica schist overgrew pervasive white mica
fabrics and were rotated to form and δ-type clasts during SCSZ
operation (Fig. 5a, cf. Passchier and Simpson, 1986).
Top: island-scale map illustrating major lithologies. Bottom:
Detailed map of study area with sample collection sites, the entire area is
affected by the broad, south-directed South Cyclades Shear Zone (SCSZ).
Locations observed with overprinting narrow, north-directed shear zones are
indicated by blue stars, diagrams after Yeung (2019). Detailed structural maps of areas marked with blue stars are in the Supplement.
Microstructures analysis in the Port Beach tectonic slice. (a)
Garnet porphyroblasts with δ-type pressure shadows in the Port Beach
garnet–mica schist (IO18-04). (b) Two generations of overprinted white mica
deformation fabrics, with new-grown (wm3) layer-parallel phengite
overprinting the “lens”-shaped (wm2) phengite which has its mineral cleavage
oblique to the fabric. (c) The plot of Si content illustrating the presence
of phengite and muscovite in the Port Beach augengneiss (IO18-01).
Two generations of white mica were observed in this unit, including
pre-mylonite porphyroblasts (now present as muscovite fish with dynamically
recrystallised rims) and the younger, recrystallised phengite (separated into
wm2 and wm3 based on their overprinting relations) that intergrew with
dynamically recrystallised K-feldspar and quartz (Fig. 5b). Silicate content
of the phengite deformation fabric is ∼3.40–3.45 Si a.p.f.u. (atoms per formula unit)
(Fig. 5c). This, along with the mineral assemblage of
quartz ± garnet ± potassium
feldspar ± hornblende ± white mica (phengite) ± biotite,
suggests P–T conditions of 1.8–2.2 GPa and 500–600∘C
based on calculations in Massonne and Schreyer (1987), Patrick (1995), Velde
(1967) and Kamzolkin et al. (2016). The small garnet blasts are preserved in
low-strain zones, particularly adjacent to pull-aparts marked by quartz-filled
voids (Fig. 5b). Amphibolite facies may have taken place in Hercynian time,
but during the Late Cretaceous it appears that the Port Beach tectonic slice
was subjected to high-pressure eclogite facies conditions.
The structurally mid-level garnet–mica schist tectonic slice
Field observations in this garnet–mica schist slice and in the top part of the
underlying augengneiss core identified evidence for multiple alternating and
overprinting deformation events such as recumbent folds overprinted by
extensional shear zones. The effects of the (here) N–S striking SCSZ fabric is
pervasive, and most of the early fabrics recrystallised during this
extensional episode. Nevertheless, relicts of earlier fabric are observed in
low-strain zones beside large porphyroclasts. Samples collected from the
structurally deeper level of this tectonic slice in the north Mylopotas
headland preserved the most complex mineral growth and micro-deformation
history. We note that metabasite was observed sporadically in the augengneiss
basement, with mineral assemblages that suggest it was subject to transitional
greenschist–blueschist metamorphism during the Δ1D event of Forster
et al. (2020). The protolith for such metabasite pockets is likely to have
been intermediate-mafic intrusive dykes that folded and deformed with the
country rock (see structural maps in the Supplement). A late-developed,
intense north-sense shear zone defines the structural contact between the
garnet–mica schist and the augengneiss in this locality. Fluid associated
haematite nodes are found in the top 3 m of an intense shear zone at
the contact between the juxtaposed garnet–mica schist tectonic slices and the
augengneiss core.
The four samples presented in Table 2 have garnet porphyroclasts recording
multiple mineral growth events and preserved earlier fabrics as inclusions.
Samples IO17-03 and IO17-04 retained the earliest formed garnets (some of which
are large, exceeding 2–3 cm in diameter). Samples IO17-03, IO17-04 and
IO17-05 preserved different stages of micro-tectonic events during SCSZ
operation. The larger, first-generation (1–2 cm diameter) garnet
porphyroblasts are intact in IO17-03, fragmented during shear zone operation
in IO17-04 and acting as porphyroclasts during deformation in IO17-05.
Relicts of earlier fabrics are preserved in the low-strain zone behind garnets
in IO17-03 and IO17-04 and are microstructurally distinguishable, whereas
fabrics in sample IO17-05 are almost completely reset by the SCSZ. IO18-05 is
collected from a fold hinge of a recumbent fold (M1 folding) that is
overprinted by the SCSZ (D2 crustal stretching), and it represents the
structurally lowest level of this tectonic slice. Haematite nucleating on the
deformation fabric is observed, and relicts of earlier deformation fabrics
with rutile are preserved as inclusions in the garnet porphyroblasts.
Mineral chemistries with the three generations of garnet growth recognised in
sample IO17-03 are chemically similar to almandine (see Supplement – electron
microprobe analysis (EPMA)) but are iron enriched and calcium depleted with
slightly higher magnesium content compared to end-member
almandine. Dynamically recrystallised, south-sense white mica fabric wraps
around the larger (2–3 cm diameter) garnet porphyroblasts, with
second-generation garnets growing over this fabric. The two younger garnet
growth events are close in time, producing crystals of different size: the
2–3 mm diameter crystals with uniform colour and the 5–8 mm
diameter garnets have a zoned mineral growth (Fig. 6a and b). Observation on
the two types of crystals identify the light red (Ca-depleted, Mn-enriched)
garnets as the first growth event, followed by the second growth event
producing black (Mn-depleted, Ca-enriched) garnets (Fig. 6c; see
Supplement). Although the later greenschist facies is pervasive across the
outcrop, with chlorite overprinting the south-directed white mica deformation
fabric, traces of rutile crystals “floating” in the relicts of earlier
(pre-SCSZ) fabric in the low-strain zone are preserved in IO17-03
(Fig. 6d). This also implies a higher-pressure history (potentially eclogite
facies) than previously recognised.
Microstructure analysis in the garnet–mica schist tectonic slice
(sample IO17-03). (a) White-mica-dominated deformation fabric with minor
biotite relict of early fabric surrounds large 2–3 cm diameter garnets,
with younger 2–3 mm garnets grown on the deformation fabric. (b) Thin
section under plane-polarised light: two types of small, second-generation
garnets with different chemical compositions are identified (see
Supplement – EPMA analysis results). (c) A slightly larger
(∼4mm diameter) second-generation garnet with a zoned
crystalline texture (thin section under plane-polarised light). (d) Magnified view of the green box in (b), under cross-polarised light a euhedral
second-generation garnet that grew into a quartz foam texture with relict
rutile floating in the void space.
Sample IO18-05, garnet–mica schist collected in the fold hinge of
the earliest fold (overprinted by the south-directed shear zone) in the
mid-level garnet–mica schist tectonic slice. (a) A garnet porphyroblast
preserving relicts of earlier deformation fabrics as inclusions and
developed a skeletal structure once it reached the Al-depleted zone. (b)
Rutile inclusions and albite exsolution trails observed in the garnet
porphyroblast. (c) A Si-content plot indicating the presence of two phengite
groups in the garnet–mica schist.
The earliest microstructure observed was within garnet porphyroblasts, rotated
during shear zone operation. This could be inferred from the oblique angle
between white mica–rutile inclusions (in IO18-05) and the recrystallised
groundmass (Fig. 7a, showing the core of a garnet porphyroblast). As the
inclusions were fine grained, energy-dispersive X-ray spectroscopy (EDS)
analysis was used to confirm the presence of rutile in the included
fabric. Electron microprobe analysis (EPMA) identified the chemical
composition the garnet porphyroblasts (5–8 mm diameter) as between
almandine and grossular, nucleated on Al-rich white mica during operation of
an early shear zone, and continuing to grow during deformation until they
reached Al-depleted, foam-textured quartz in pressure shadows (Supplement – EPMA analysis results). Upon reaching the foam-textured quartz, the fluids
then corroded grain boundaries, allowing the new-grown garnet to develop a
skeletal structure in which the original foam texture in the incorporated
quartz grains can still be recognised (Fig. 7b, bottom right
corner). Late-stage (first-order) grey albite grew in exsolution trails
preserved across the garnet porphyroblasts, implying decompression as the
garnet porphyroclast fractured during shear zone operation. (Fig. 7b,
Supplement – EPMA analysis results).
Dynamic recrystallisation of white mica (phengite) and quartz in the
groundmass of IO17-03 and IO18-05 occurred synchronously during shear zone
operation. The Si content of phengite in samples IO17-03 and IO18-05 suggests
that the phengite grew under P–T conditions up to 450–500∘C
(based on the presence of garnet and biotite) with pressure in the range
0.7–1.7 GPa based on calculations in Massonne and Schreyer (1987),
Patrick (1995), Velde (1967) and Kamzolkin et al. (2016) (Fig. 7c). We
therefore suggest that the structurally mid-level garnet–mica schist tectonic
slice also recorded a complex history of deformation and metamorphism with
evidence of high-pressure transitional amphibolite–eclogite facies
metamorphism.
The augengneiss core: the structurally lowest level
The structurally deepest augengneiss core of the Ios basement terrane is
characterised by large (0.3–1.0 cm) K-feldspar xenocrysts preserved
as porphyroclasts. Rocks in this locality were deformed by the south-directed
South Cyclades Shear Zone (SCSZ) then variably overprinted by narrow
north-directed shear zones. Occasionally, single hornblende porphyroclasts
wrapped by a south-sense white-mica shear fabric could be observed (e.g.,
Fig. 8a, in a thin-section cut parallel to the stretching lineation).
Pre-deformation hornblende was also observed in low-strain zones adjacent to
these K-feldspar porphyroclasts (Fig. 8b). K-feldspar porphyroclasts
surrounded by dynamically recrystallised white mica and quartz in these sample
were fractured by shearing, with recrystallisation at the edges (Fig. 8b). The
youngest microstructures observed in these samples are quartz filled
cracks. The augengneiss basement records evidence of recumbent folding during
crustal shortening, followed by ductile stretching under a south-directed
shear zone. All of this occurred before the augengneiss was juxtaposed against
the garnet–mica schist slice by an intense north-directed shear zone.
Microstructures analysis on the south Mylopotas headland
augengneiss (AG03-03, AG03-04, AG03-05). (a) An older hornblende preserved
as large porphyroclasts wrapped by younger, recrystallised white. (b)
K-feldspar porphyroclasts with minor recrystallisation limited to their
boundaries, and a hornblende xenocryst preserved in a low-strain zone
(AG03-04). (c) K-feldspars porphyroclasts overprinted by both the earlier
south-directed shear zone and the younger north-directed shear zone, forming
“micro boudinage” structures (AG03-03).
White mica age spectra from the structurally mid-level garnet–mica
schist unit (IO17-03, IO17-05) produced in the Late Cretaceous ages, from mica
grown during the Asteroussia event.
Argon geochronology
To provide time constraints on mineral growth events and deformation observed
across the three tectonic slices, new Ar40/Ar39 geochronology data were collected using furnace-based step heating experiments conducted under
ultra-high vacuum (UHV) conditions. These enabled new data that allowed
recognition of Late Cretaceous Asteroussian ages in the garnet–mica schist
mid-level unit where relicts of earlier, rutile-containing fabrics are
preserved. Argon geochronology was performed on white micas from deformation
fabrics and on the K-feldspar porphyroclasts, with results summarised in
Table 2. White micas with grain sizes ranging from 250 to 420 µm
were used as microstructural analysis and identified the relicts of the earliest
fabric to be of larger grain sizes compared to the dynamically recrystallised
Alpine deformation fabrics (refer to the Supplement on the 40Ar/39Ar
analytical technique). No new analysis is performed on the two augengneiss
samples AG03-03 and AG03-05 collected at the augengneiss core, but the data
previously published by Forster and Lister (2009) were re-examined in order to
link microstructures observed with the reported Late Cretaceous dates.
The age spectra produced varied in their character depending on the structural
character and rock type. For example, the morphologies of the argon spectra
obtained from the garnet–mica schist are different and distinct in comparison
with those obtained from the augengneiss. The phengitic white micas from the
thick garnet–mica schist slice produced spectra with a characteristic
“hump-shaped” partial plateau, whereas age spectra from phengitic white mica
in the underlying augengneiss generally produced spectra with a partial
plateau rising to a peak in the final heating steps. The Late Cretaceous
Asteroussia ages are always preserved in phengitic white mica, and since this
appears to be highly retentive of radiogenic argon, these are likely to be
growth ages and hence key to identifying older Asteroussian fabrics
overprinted by younger Alpine events. Previous research suggested that the
later-formed shear zones operating in this area operated in the Argon Partial
Retention Zone (Baldwin and Lister, 1998; Forster and Lister, 2009), but this
was on the basis that it had been assumed that all the white mica was
muscovite, which is not correct. The complex age spectra preserve and record
the effect of multiple deformation and metamorphic mineral growth events, but
they are preserved only because phengitic white mica (especially under high-pressure conditions) is extremely retentive of argon (Lister and Baldwin,
1996; Warren et al., 2012).
Argon geochronology on white mica and K-feldspar grain separated from the
three tectonic slices in the Ios basement terrane yielded age clusters in
Early–Middle Jurassic, Late Cretaceous, Eocene–Oligocene and
Oligocene–Miocene time (Table 2). However, evidence for Jurassic and
Cretaceous ages is exclusively restricted to argon populations retained in
phengite, or, in the case of IO18-01, to the large muscovite fish. All white
mica analysed yielded Arrhenius plots that unequivocally demonstrate both
phengite and muscovite components, e.g., IO17-05 in the garnet–mica schist,
and AG03-03 in the augengneiss (Figs. 9 and 10; see corresponding figures in
the Supplement). The phengitic components produce significantly high
activation energy estimates, in the range
103–115 kcalmol-1 (431–481 kJmol-1) compared to estimates from the
muscovite domain, in the range 54–61 kcalmol-1
(226–255 kJmol-1) (Fig. 10; see corresponding figures in the
Supplement). The estimated retentivity of the phengite implies that the ages
measured are growth ages, since metamorphic temperatures were less than the
inferred closure temperatures from the Arrhenius plots. Therefore, it appears
that we have been successful in being able to directly date microstructures
produced during the Late Cretaceous Asteroussia event.
White mica age spectra from the structurally lowest augengneiss
core (AG03-03, AG03-05). The complex age spectra are a result of multiple
argon populations degassing at different temperature during the step-heating
experiment. The augengneiss basement was subjected to multiple deformation
events, but a significant argon population is derived from phengite with
Late Cretaceous ages preserved. The argon geochronology data were published
in Forster and Lister (2009) and re-analysed in this study.
The garnet–mica schists that produced the Late Cretaceous Asteroussia ages
were collected in the northern headland of Mylopotas Beach, Ios
(Fig. 4). We have already noted that microstructural analysis of the
garnet–mica schist IO17-03 and IO17-05 demonstrated multiple episodes of white
mica growth. The older grains in the deformation fabric are 180 to
450 µm in diameter, whereas the younger grains developed during or
after later shear zone operation are elongate with dimensions ranging from 50 to
90 µm. Note that the older-generation phengitic white micas
(355–450 µm grains) were tediously hand-picked for this
sample. The 40Ar/39Ar results suggest several different gas
populations retained in the crystal lattice, with a younger gas population in
the less retentive domain and an older gas population in the more retentive
domain that dominated gas release. The older argon population accounted for
90 % argon released in IO17-03 white mica and created a partial
plateau (“hump”) with peak minimum age of 76.9±0.7Ma in the
phengitic part of the age spectrum (Fig. 9a; see corresponding figures in
Supplement). The younger gas population that accounts for the 5 %
of initial argon release comes from muscovite formed later in the geological
history, during operation of the SCSZ. However, white mica from the relicts of
earlier fabrics in IO17-05 preserved an older argon population with peak
minimum age of 81±0.6Ma. A younger gas population in the less
retentive domain of the earlier white mica fabric in IO17-05 record an age of
59±1.5Ma, which is comparable with estimates for the timing of
the Δ1A and Δ1B Alpine events (Forster et al., 2015; Huet
et al., 2009).
Samples AG03-03 and AG03-05 were collected from the augengneiss core, and
40Ar/39Ar geochronology on isolated white mica deformation
fabrics were performed in the study reported in Forster and Lister (2009). The
two rocks are microstructurally similar; fabrics underwent minor
recrystallisation during south-directed then north-directed shear zone
operation. We reanalysed the diffusion experiment result in this study as
white mica grain separates from the two samples produced Late Cretaceous dates
(Forster and Lister, 2009). Application of the method of asymptotes and limits
on the AG03-03 white mica age spectrum yielded a range of ages from
70.4–74.0 Ma (Fig. 10a; Forster and Lister, 2004) for phengitic white mica (Fig. 10a; see
corresponding Supplement). White mica deformation fabrics in augengneiss
AG03-05 show an upper limit at 72±0.6Ma and a lower limit at
68.3±0.3Ma in a gas release of the more retentive domain,
representing the minimum and maximum ages of a single Late Cretaceous event
respectively (Fig. 10b). The older ages in the age spectra may represent even
older relict fabrics. From these data it is evident that the structurally
mid-level garnet–mica schist and the underlying augengneiss basement were
subjected to complex deformation history with multiple events occurring from
Early Cretaceous to Miocene time.
White mica and K-feldspar differ in their 40Ar/39Ar systematics
and grow and respond differently to deformation. K-feldspar grain separates
were collected from all augengneiss samples in an unsuccessful attempt to
pinpoint the microstructure(s) responsible for the Late Cretaceous date
reported in the white mica. Forster et al. (2014) reported that K-feldspars
required analysis with isothermal steps so as to recognise contamination at
each temperature increase in the step heating procedure (i.e., isothermal
steps being two or more heating steps at the same temperature). The first step
is referred to as a cleaning step and is not included in the interpretation of
the spectrum. This same methodology is used on the K-feldspar analysis in this
study.
(a) A K-feldspar age spectrum from the structurally lowest
augengneiss core (with isothermal cleaning steps removed). The origin of the
younger part of the age spectrum is discussed by Forster and Lister (2009).
The Late Cretaceous age is preserved in the retentive core domains. (b) The
corresponding Arrhenius plot shows two diffusion domains with significantly
different activation energies.
In AG03-03, Forster and Lister (2009) observed larger K-feldspars
(porphyroclasts; 2000–6500 µm) and small K-feldspar grains
(500–700 µm) interspersed between aligned white mica grains that
recrystallised during later deformation. Step-heating experiments on the
K-feldspar grain separates (including both porphyroclasts and small grains)
from AG03-03 produced a saddle-shaped apparent age spectrum with a lower limit
at ∼13±0.1Ma (Fig. 11a). The last argon release steps
produced a peak at 84.5±0.8Ma, comparable to the date obtained
from white mica from the same sample (Fig. 10a). The Arrhenius plot of
K-feldspar in sample AG03-03 shows two distinct argon diffusion domains
(Fig. 11b, see Forster and Lister, 2010). This suggests that the K-feldspar in the south Mylopotas
augengneiss also preserved complex deformation history, with the oldest (and
most retentive domains) regrown during the Late Cretaceous event. The
Arrhenius data (Fig. 11b) show that these older domains were capable of
retaining argon at temperatures well above those recorded by the metamorphic
assemblages, implying that these are growth ages, requiring the original
potassium feldspar to have been replaced during metamorphism and/or
metasomatism by this time.
DiscussionEvidence of the Asteroussia event in the Ios lower plate
Microstructurally, our study has conclusively identified the presence of more
retentive phengite in a fabric that was later overprinted by dynamically
recrystallised white mica and quartz. The earlier metamorphic fabrics formed
under conditions that potentially reached eclogite facies. Our UHV 39Ar
diffusion experiments show that this phengite is highly retentive, allowing
preservation of the growth ages of the white mica that formed during these
earlier events. Thus, despite intense overprinting during Alpine deformation
events, the Late Cretaceous argon populations were retained. This is
consistent with the concept of an Argon Partial Retention Zone in which
mineral grains undergo some partial resetting by diffusion, but where
recrystallisation causes the most effects (Baldwin and Lister, 1998). However,
the concept of a partial retention zone is appropriate only for systems with
single diffusion domains and no variation of activation energy, which is not
the case here.
Identification of the Late Cretaceous age in the Ios lower basement has been
interpreted as a result of mixing (e.g., Andriessen et al., 1987): in other
words, defining these dates as “intermediate” ages due to excess radiogenic
argon or simultaneous degassing of the Alpine mica and the older Hercynian
micas. However, here we have shown that these ages represent a period of Late
Cretaceous deformation and metamorphism. Therefore, the Ios basement may
indeed be part of the Asteroussia terrane. However, pressure–temperature
estimates from phengites in the Ios lower plate record high-pressure
conditions, contrary to what has been observed in Asteroussia klippen across
the Cyclades, albeit preserved at different structural levels. This suggests
that more than one set of tectonic slices may have preserved the Asteroussian
ages, and we have already pointed to the role that tectonic shuffling may play
in producing such variation. It is important in this aspect that the Ios data
are the first report of Asteroussia ages in a terrane of unmistakeably
Gondwanan affinity (Keay and Lister, 2002).
There may be an earlier Hercynian history: the earliest reported argon age in
Ios is a single K/Ar hornblende date reported to be post-Hercynian (268±27Ma) by Andriessen et al. (1987) and Flansburg et al. (2019).
However, based on the peak metamorphic P–T conditions documented across the
Cyclades, it may be that the Asteroussian terrane slices record a variety of
metamorphic pressure conditions (Table 3, and references therein). Rocks from
the basement slices on Ios suggest the occurrence of high-pressure–medium-temperature conditions based on the microchemistry preserved in relicts of
earlier deformation fabrics.
Peak metamorphic condition of the Asteroussia event across the
cyclades.
IslandPublished studiesSample details/methodologyPeak metamorphic conditionTinosPatzak et al. (1994) (as cited in Be'eri-Shlevin et al., 2009)Interlayered amphibolite–paragneisssequence in Akrotiri unit650–750 MPa 530–610 ∘CDonoussaKolodner et al. (1998)P–T estimates on garnet–sillimanite–biotite–quartz assemblage observed in pelitic rocks.Core of garnet 400–500 MPa 600–650 ∘CDistinct chemical zoning of garnets allowed P–T calculation in core and rim respectivelyRim of garnet 250–350 MPa 550–580 ∘CAnafiBe'eri-Shlevin (2009)EPMA analyses of garnet–biotite pairs from garnet-biotite paragneiss sample that occur as thin (1–2 m thick) layers within the structurally intermediate level of the Asteroussia Unit. Garnet–biotite temperatures were calculated using the equation of Ferry and Spear (1978, as cited in Be’eri-Shlevin et al., 2009).Core of garnet & biotite ∼720±50–740±50∘C 200–600 MPa Rims of garnet & biotite 634±50–650±50∘C 200–600 MPaSample collected from a massive amphibolite exposure in the structurally intermediate level of the Asteroussia Unitedenite–tremolite (ed–tr) reactions677–726 ∘C 200–600 MPaedenite–richterite (ed–ri) reactions605–643 ∘C 200–600 MPaCreteSeidel et al. (1981)Peak metamorphism P–T conditions estimated from critical mineral assemblage of the outcrop of a variegated series consisting of: tholeiitic ortho-amphibolites, para-amphibolites, andalusite and sillimanite-cordierite-garnet bearing mica schists, calc-silicate rocks, and marbles.400–500 MPa maximum temperature ∼700∘CAnderson and Smith (1995) (as cited in Langosch et al., 2000)Al-in- hornblende barometer Granodiorites of eastern Crete100–200 MPa Maximum temperature = 700 ∘CGranites and granodiorites of central Crete250–400 MPa Maximum temperature = 700 ∘CKoepke and Seidel (1984) (as cited in Langosch et al., 2000)Peak metamorphism P–T conditions estimated from metamorphic assemblages of quartz – plagioclase – K-feldspar – sillimanite – biotite – garnet – cordierite in pelitic paragneisses at central Creteupper amphibolite facies: 400–600 MPa 650–700 ∘CLangosch (1999) (as cited in Langosch et al., 2000) Calculated by thermobarometric calibrations of Bhattacharya et al. (1988, 1992), Dwivedi et al. (1998), Koziol and Newton (1988) and Holland and Blundy (1994), all as cited in Lamngosch et al. (2000) Peak metamorphism P–T conditions estimated from metamorphic assemblages of(1) quartz – muscovite – chlorite – garnet – andalusite – plagioclase and680–730 ∘C 500–600 MPa(2) quartz – muscovite – biotite – staurolite – andalusite – plagioclase observed in metapelites of Asteroussian tectonic sliceslower amphibolite facies: ∼550∘C 300 MPaTectonic implications
The nature of the tectonic processes that affected the evolution of the
terranes accreted by the Hellenic subduction zone remains controversial, e.g.,
comparing the papers by Forster and Lister (2009), Forster et al. (2020) to
that written by Huet et al. (2009) and (2011). However, the
polemic seems misguided. The architecture of Tethyan orogenic belts, the
Hellenides included, invariably involves a nappe stack or a terrane stack, and all
terrane stacks are created by thrusting. However, most if not all terrane
stacks are also modified by later episodes of extension (e.g., as in Forster
and Lister, 2009) leading to tectonic shuffling. It is no different in the
Cyclades. The Cycladic archipelago preserves the results of the destruction of
an extensive terrane stack that extended from the Hellenides in Greece to the
Taurus Mountains in Turkey (Gautier and Brun, 1994a, b; Kempler and Garfunkel,
1994; McKenzie, 1977; Taymaz et al., 1991). The debate as to the nature of
exhumation processes will not be resolved by a sole focus on the Cycladic
eclogite–blueschist belt, as demonstrated in this paper.
The key questions surround the evolution of the terrane stack overall, rather
than the details of the exhumation of an individual tectonic slice. The
extrusion wedge (or forcible eduction) model suggests constant compression,
resulting in the squeezing of softer material, so that it is extruded to the
surface (Forster and Lister, 2008; Xypolias and Koukouvelas, 2001). The
competing hypothesis, known as the tectonic mode switch or tectonic shuffle
zone model, considers that thrust slices are exhumed by periods of crustal
extension that take place in between episodes of crustal shortening caused by
individual accretion events (Forster and Lister, 2009). Dispute arises
because of the focus on the exhumation of the Cycladic eclogite–blueschist
terranes, whereas the continuing nature of the orogenic process means that
(without question) the subduction megathrust had to have episodically leapt
southward every time a new terrane was accreted (e.g., Lister et al., 2001;
Ring et al., 2007; Huet et al., 2009; Forster and Lister, 2009). As the
African plate migrated northward, terranes were first subducted, then sliced
from the subducting lithosphere by the advancing subduction megathrust and
thus accreted to the terrane stack (e.g., Lister et al., 2001; Lister and
Forster, 2009).
For Ios, the question is how rollback of the subducting slab was able to throw
the overriding terrane stack into horizontal extension immediately after the
accretion of the Cycladic blueschist onto the Gondwanan basement from which
the Asteroussian terranes were derived (Fig. 4), in particular given the
requirement thereafter of a massive southwards leap of the outcrop of the
active subduction megathrust. Previous work (e.g., Forster et al., 2020) has
suggested that the Cycladic blueschist belt had already been largely exhumed
before it was thrust over the Ios basement terrane in Late Eocene time (from
∼38Ma, Fig. 4a). A first period of extensional tectonism formed
the Ios metamorphic core complex, and this had commenced by ∼35Ma, accelerating by the time of the Eocene–Oligocene
transition. A second period of extensional tectonism then ensued, after the
Oligocene–Miocene transition, with extreme lithospheric extension triggering
a major magmatic event, with intrusions in and through the core of younger
metamorphic core complexes across the Cyclades.
The Ios basement has been argued to be autochthonous, moving with Africa, and
part of Gondwana (Flansburg et al., 2019; Keay et al., 2001; Keay and Lister,
2002). Its accretion to the terrane stack is therefore likely to have been an
event with considerable tectonic significance. The magnitude of the southward
leap of the subduction megathrust is thus unlikely to have been accomplished
without the development of a new lithosphere-scale structure. There are two
end-member options: one requiring that the slab peels free from the subduction
megathrust (Fig. 3, using the slab peel hypothesis discussed by Brun and
Faccenna, 2008) while the other requires a subduction jump and slab break-off
(Fig. 4, cf. von Blanckenburg and Davies, 1995). Although the slab-peel model
is consistent with enhanced heat flow during crustal stretching after the
accretion event, such a model requires the asthenosphere to be exhumed to such
shallow levels as to require significant partial melting of the uplifting
asthenosphere, which would a period of widespread basaltic volcanism, with
volumes comparable to those observed in some large igneous provinces. Such
effects were not observed in the Cyclades. Sizova et al. (2019) also showed
the “peel-off” model (Brun and Faccenna, 2008) to be unlikely in the Aegean
region.
An alternative model involving slab necking (or boudinage) and break-off must
therefore be considered (e.g., Fig. 4). This (provisional) three-staged “slab
break-off” model more accurately describes Aegean tectonics by addressing how
the terrane stack was subjected to overall stretching with some evidence of
melting such as plutonic intrusions in the centre of metamorphic core
complexes. This model also requires significant magmatism, but in consequence
of fluids rising from a devolatising slab which would lead first to crustal
magmatism, such as the I-type granite of Ios, and later to the appearance of
arc volcanoes, as on Thera. The necking and eventual break-off of the
subducting slab and formation of a new subduction zone (Fig. 4d) might be of
sufficiently small scale to escape observation in models based on P-wave
tomography.
Unresolved issues
We do not understand why the Asteroussia event is recorded in the top-most
slices of the terrane stack outcropped in other Cycladic islands but is found
only in the lower slice in Ios. Such architecture implies that the Cycladic
eclogite–blueschist tectonics slices are “sandwiched” between tectonic
slices affected by the Asteroussia event, whereas on Crete the Asteroussia
units are juxtaposed above the Vatos unit, the Arvi unit, the Pindos unit and
the Tripolitza unit (e.g., Bonneau, 1984; Flansburg et al., 2019; Kneuker
et al., 2015; Langosch et al., 2000; Martha et al., 2017; Martha et al., 2016;
Palamakumbura et al., 2013; Seidel et al., 1976; Zulauf et al., 2002). This
must have occurred sometime between mid-Oligocene–early Miocene
time. Laterally, the unit is connected to the eastern Alps in the west and the
Lycian ophiolite nappes, the Menderes Massif and the Sakarya Zone in Turkey
(van Hinsbergen et al., 2020). Further work is required to validate the
tectonic-shuffling hypothesis which is capable of explaining these
observations.
Some authors suggest that tectonic slices outcropping on islands in the
northwest (Andros, Tinos, Syros) are different to those on other islands such
as Anafi, Nikoria, Donoussa, Ikaria and Crete (Altherr et al., 1994; Langosch
et al., 2000; Martha et al., 2016). Arguments arise due to the difference in
dates obtained (despite all being Late Cretaceous) and different results for
geothermobarometry across islands with different lithologies and metamorphic
facies (Kolodner et al., 1998; Langosch et al., 2000; Patzak et al., 1994;
Seidel et al., 1976, 1981; Yeung, 2019). Research in the upper
and middle tectonic units in Tinos produced dates at 90–100 Ma and a
peak metamorphic P–T estimate of 120 MPa at 450–500 ∘C
(Avigad and Garfunkel, 1989; Avigad and Garfunkel, 1991; Bröcker and
Franz, 1998; Patzak et al., 1994), whereas studies on Donoussa and Crete
produced younger ages at 70–80 Ma and peak metamorphic P–T conditions
at 300–600 MPa and 600–730∘C (Be'eri-Shlevin et al.,
2009; Keay and Lister, 2002; Kolodner et al., 1998; Langosch et al., 2000;
Seidel et al., 1976).
Our study reports metamorphic conditions with higher pressure, despite
producing similar dates. Although the presence of phengite is widespread
across the Ios lower plate, the highest pressures are inferred only in the
garnet–mica schist unit and the Port Beach tectonic slice. With no evidence of
higher pressures in the underlying augengneiss unit, it is possible that a
more complex deformation and metamorphic history has been recorded in these
intermediate slices in the Ios terrane stack. These observations also reflect
on a possible distinction between European and Gondwanan terranes, with
evidence mostly preserved in the Alps and in the Pelagonian zone of Greece
(Pourteau et al., 2013; Porkoláb et al., 2019; Regis et al., 2014;
Thöni, 2006). Brown et al. (2014) reports evidence of Late Cretaceous
intracontinental shear zone deformation across Africa, thus demonstrating that
the ∼70–80 Ma age is not limited to northern Tethys. Detrital
zircon (DZ) analysis on pre-plutonic metasedimentary rocks in the Ios lower plate
by Flansburg et al. (2019) pushes tectono-magmatic histories of the southern
Cyclades further in time to the early Cenozoic. They noted a striking resemblance
between their DZ age spectra from Ios lower plate to exposures on Crete,
northern and central Peloponnese, the northern Hellenides, and the
siliciclastic cover sequence of the Menderes massif in western Turkey
(Flansburg et al., 2019). Comparing these Ios DZ age spectra to those from
northeast Africa and Arabia, they confirmed that the Cycladic basement terrane
(outcropped in the Ios lower plate) has a distinct peri-Gondwanan affinity
(Flansburg et al., 2019). This led them to propose a tectonic model where the
terrane was located along the northern margin of Gondwana in the early Paleozoic
and experienced pluton emplacement between ∼335 and ∼305Ma in an arc setting (Flansburg et al., 2019).
Tectonic reconstructions by van Hinsbergen et al. (2020) demonstrated that
major continental-scale events occurred across Eurasia and Gondwana from Late
Jurassic–Late Cretaceous time. These global tectonic events involve
continental-scale deformation such as the formation of the Alpine Tethys with
microcontinents tearing from the south coast of Europe. In their
reconstruction model, the Ios basement, along with other tectonic units in the
Cycladic islands and Crete, are all part of a subducted Greater Adria
continental ribbon. While our island-scale study cannot contribute to the
discussion on whether Greater Adria was a single continental landmass or made
up of several large islands, it is evident that Late Cretaceous deformation is
widespread in both the European and Gondwana terranes.
Distinguishing European versus Gondwanan terranes in the Central Aegean and
greater Mediterranean area will remain difficult. One central argument is the
number of oceans present in the “greater Tethys seaway” between Europe,
Africa and potentially Adria at Mesozoic and associated tectonic evolution
(e.g., Channell and Kozur, 1997; Kilias et al., 2010; Robertson et al.,
2013). This argument mainly concerns the paleogeography of the Pelagonian unit
outcropped in mainland Greece. It is thus of interest that evidence for Late
Cretaceous ages is reported from white mica deformation fabrics isolated from
the northern end of the upper Pelagonian unit (e.g., Kilias et al., 2010;
Robertson et al., 2013). The Pelagonian unit may have been a continental
ribbon (or micro-continent) separating two Tethyan realms: the Vardar Ocean in
the northeast and the Pindos (or even Cyclades) Ocean in the southwest (e.g.,
Channell and Kozur, 1997; Robertson et al., 2013). Such models imply high-pressure metamorphism in the Pelagonian unit as the result of the attempted
subduction of the continental ribbon (Robertson et al., 2013). Other
reconstructions consider the Pelagonian unit as the eastern-most unit of a
continental Adria terrane, adjacent to a single northeastern oceanic basin
(the Vardar Ocean) (e.g., Bortolotti et al., 2013; Ferriere et al., 2012;
Kilias et al., 2010; Palamakumbura et al., 2013). These researchers disagree
with the concept of a distinct Pindos Ocean both in Triassic and in Jurassic
time (Kilias et al., 2010).
Conclusions
Our study reports evidence of a Late Cretaceous Asteroussia event
(70–80 Ma) in the originally Gondwanan lower plate of Ios. Accretion
of the Asteroussia terrane is a major event in the Aegean tectonic
history. This required a (250–300 km) southward jump of the
subduction megathrust. Renewed rollback after the accretion event triggered
Oligocene extension and facilitated the exhumation of the Asteroussia terrane
within the core of the Ios metamorphic core complex.
Data availability
40Ar/39Ar geochronology results of two augengneiss samples
(AG03-03, AG03-05) were published in Forster and Lister (2009) (https://agupus.onlinelibrary.wiley.com/doi/pdf/10.1029/2007JB005382; last access: 26 August 2021) and
re-examined in this study. All new data collected in this study and
presented in this article are provided in the text and in the Masters thesis of
Sonia Yeung submitted for her Masters programme to the Research School of
Earth Sciences, Australian National University.
The supplement related to this article is available online at: https://doi.org/10.5194/se-12-2255-2021-supplement.
Author contributions
All authors contributed to the writing of the paper and its
conceptualisation. The paper extends part of a Master's thesis by Sonia
Yeung, supervised by Marnie Forster and Gordon Lister.
Competing interests
The authors declare that they have no conflicts of interest.
Disclaimer
The article includes a minor part of the Masters thesis of Sonia Yeung
submitted for her Masters programme to the Research School of Earth
Sciences, Australian National University.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The authors acknowledge the microprobe mineral chemistry analysis was
performed in the facilities, with the scientific and technical assistance in
Microscopy Australia at the Centre of Advanced Microscopy, The Australian
National University. Sample Irradiations for 40Ar/39Ar
geochronology were facilitated by the University of California Davis McClellan
Nuclear Research Centre, CA, US. Argon analyses and microstructure analyses
were performed at the Research School of Earth Sciences Laboratories at the
Australian National University. Davood Vasegh in the Argon Lab provided
technical assistance for the step heating experiments, Shane Paxton in the
mineral separation facility provided technical assistance for sample
preparation. Step-heating experimental results for 40Ar/39Ar
geochronology were analysed using programmes eArgon and
MacArgon developed by G. S. Lister (http://rses.anu.edu.au/tectonics/programs/, last access: 26 August 2021).
Financial support
Research support were provided by the Australian Research Council Discovery
Project (grants numbers: DP120103554 “A unified model for the closure
dynamics of ancient Tethys constrained by Geodesy, Structural Geology, Argon
Geochronology and Tectonic Reconstruction” and LP130100134 “Where to find
giant porphyry and epithermal gold and copper deposits”). Sample
Irradiations were paid by the Research School of Earth Science argon
facility, Australian National University and facilitated by the University of California Davis McClellan Nuclear Research Centre, CA, US.
Review statement
This paper was edited by Mark Allen and reviewed by Franz Neubauer and one anonymous referee.
References
Altherr, R., Kreuzer, H., and Lenz, H.: Further Evidence for a Late Cretaceous Low-pressure, Chem. Erde-Geochem., 54, 319–328, 1994.
Andriessen, P., Banga, G., and Hebeda, E.: Isotopic age study of pre-Alpine rocks in the basal units on Naxos, Sikinos and Ios, Greek Cyclades, Geol. Mijnbouw, 66, 3–14, 1987.
Avigad, D. and Garfunkel, Z.: Low-angle faults above and below a blueschist belt – Tinos Island, Cyclades, Greece, Terra Nova, 1, 182–187, 1989.
Avigad, D. and Garfunkel, Z.: Uplift and exhumation of high-pressure metamorphic terrains: the example of the Cycladic blueschist belt (Aegean Sea), Tectonophysics, 188, 357–372, 1991.
Baldwin, S. L. and Lister, G. S.: Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone, J. Geophys. Res.-Sol. Ea., 103, 7315–7336, 1998.Be'eri-Shlevin, Y., Avigad, D., and Matthews, A.: Granitoid intrusion and high
temperature metamorphism in the Asteroussia Unit, Anafi Island (Greece):
Petrology and geochronology, Israel J. Earth Sci., 58, 10.1560/IJES.58.1.13, 2009.
Bonneau, M.: La Nappe Metamorphique de l'aAsteroussia, Lambeau d'affinities
pelagoniennes charrie jusque sur la zone de Tripolitzade la Crete Moyrnnr,
Comptes rendus de l'Académie des Sciences, France, Vol. 275, Num. 0021, 2303–2306, 1972.
Bonneau, M.: Correlation of the Hellenide nappes in the south-east Aegean and their tectonic reconstruction, Geol. Soc. Spec. Publ., 17, 517–527, 1984.
Bortolotti, V., Chiari, M., Marroni, M., Pandolfi, L., Principi, G., and Saccani, E.: Geodynamic evolution of ophiolites from Albania and Greece (Dinaric-Hellenic belt): one, two, or more oceanic basins?, Int. J. Earth Sci., 102, 783–811, 2013.
Bröcker, M. and Franz, L.: Rb–Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity, Geol. Mag., 135, 369–382, 1998.
Bröcker, M. and Franz, L.: Dating metamorphism and tectonic juxtaposition on Andros Island (Cyclades, Greece): results of a Rb–Sr study, Geol. Mag., 143, 609–620, 2006.
Brown, R., Summerfield, M., Gleadow, A., Gallagher, K., Carter, A., Beucher, R., and Wildman, M.: Intracontinental deformation in southern Africa during the Late Cretaceous, J. Afr. Earth Sci., 100, 20–41, 2014.
Brun, J.-P. and Faccenna, C.: Exhumation of high-pressure rocks driven by slab rollback, Earth Planet. Sc. Lett., 272, 1–7, 2008.
Channell, J. and Kozur, H.: How many oceans? Meliata, Vardar and Pindos oceans in Mesozoic Alpine paleogeography, Geology, 25, 183–186, 1997.Dürr, S., Altherr, R., Keller,J., Okrusch, M., and Seiderl, E.: The
MedianAegean Crystalline Belt: Straligraphy structure, Metamorphism,
Magmatism, in: Alps, Apennines, Hellenides. geodynamic investigation along
geotraverses by an international group of geoscientist, edited by: Cloos, H.,
Roeder, D., and Schmidt K., Schweizerbart, Stuttgart, Germany, 455–477, 1978.
Ferriere, J., Chanier, F., and Ditbanjong, P.: The Hellenic ophiolites: eastward or westward obduction of the Maliac Ocean, a discussion, Int. J. Earth Sci., 101, 1559–1580, 2012.
Flansburg, M. E., Stockli, D. F., Poulaki, E. M., and Soukis, K.: Tectono-magmatic and stratigraphic evolution of the Cycladic basement, Ios Island, Greece, Tectonics, 38, 2291–2316, 2019.
Forster, M. and Lister, G.: Detachment faults in the Aegean core complex of Ios, Cyclades, Greece, Geol. Soc. Spec. Publ., 154, 305–323, 1999a.
Forster, M. A. and Lister, G. S.: Separate episodes of eclogite and blueschist facies metamorphism in the Aegean metamorphic core complex of Ios, Cyclades, Greece, Geol. Soc. Spec. Publ., 164, 157–177, 1999b.Forster, M. and Lister, G.: The interpretation of
40Ar/39Ar apparent age spectra produced by mixing: application of
the method of asymptotes and limits, J. Struct. Geol., 26,
287–305, 2004.
Forster, M. and Lister, G.: Tectonic sequence diagrams and the structural
evolution of schists and gneisses in multiply deformed terranes,
J. Geol. Soc. London, 165, 923–939, 2008.Forster, M. and Lister, G.: Core-complex-related extension of the Aegean
lithosphere initiated at the Eocene-Oligocene transition, [data set],
J. Geophys. Res.-Sol. Ea., 114, B02401, 10.1029/2007JB005382, 2009.
Forster, M. and Lister, G.: Argon enters the retentive zone:
reassessment of diffusion parameters for K-feldspar in the South Cyclades
Shear Zone, Ios, Greece, Geol. Soc. Spec. Publ.,
332, 17–34, 2010.Forster, M. and Lister, G.: 40Ar/39Ar geochronology and the diffusion of 39Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo, Geol. Soc. Spec. Publ., 378, 117–135, 2014.Forster, M., Lister, G., and Lennox, P.: Dating deformation using crushed alkali feldspar: 40Ar/39Ar geochronology of shear zones in the Wyangala Batholith, NSW, Australia, Aust. J. Earth Sci., 61, 619–629, 2014.Forster, M., Armstrong, R., Kohn, B., Lister, G., Cottam, M., and Suggate, S.: Highly retentive core domains in K-feldspar and their implications for 40Ar/39Ar thermochronology illustrated by determining the cooling curve for the Capoas Granite, Palawan, The Philippines, Aust. J. Earth Sci., 62, 883–902, 2015.Forster, M., Koudashev, O., Nie, R., Yeung, S., and Lister, G.: 40Ar/39Ar thermochronology in the Ios basement terrane resolves the tectonic significance of the South Cyclades Shear Zone, Geol. Soc. Spec. Publ., 487, 291–313, 2020.
Gautier, P. and Brun, J.-P.: Ductile crust exhumation and extensional detachments in the central Aegean (Cyclades and Evvia Islands), Geodin. Acta, 7, 57–85, 1994a.
Gautier, P. and Brun, J.-P.: Crustal-scale geometry and
kinematics of late-orogenic extension in the central Aegean (Cyclades and Ewia
Island), Tectonophysics, 238, 399–424, 1994b.Huet, B., Labrousse, L., and Jolivet, L.: Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece), Tectonics, 28, TC3007, 10.1029/2008TC002397, 2009.
Huet, B., Le Pourhiet, L., Labrousse, L., Burov, E., and Jolivet, L.: Post-orogenic extension and metamorphic core complexes in a heterogeneous crust: the role of crustal layering inherited from collision. Application to the Cyclades (Aegean domain), Geophys. J. Int., 184, 611–625, 2011.
Kamzolkin, V. A., Ivanov, S. D., and Konilov, A. N.: Empirical phengite geobarometer: Background, calibration, and application, Geol. Ore Deposit., 58.8, 613–622, 2016.
Keay, S. and Lister, G.: African provenance for the metasediments and metaigneous rocks of the Cyclades, Aegean Sea, Greece, Geology, 30, 235–238, 2002.
Keay, S., Lister, G., and Buick, I.: The timing of partial melting, Barrovian metamorphism and granite intrusion in the Naxos metamorphic core complex, Cyclades, Aegean Sea, Greece, Tectonophysics, 342, 275–312, 2001.
Kempler, D. and Garfunkel, Z.: Structures and kinematics in the northeastern Mediterranean: a study of an irregular plate boundary, Tectonophysics, 234, 19–32, 1994.
Kilias, A., Frisch, W., Avgerinas, A., Dunkl, I., Falalakis, G., and Gawlick, H.-J.: Alpine architecture and kinematics of deformation of the northern Pelagonian nappe pile in the Hellenides, Aust. J. Earth Sci., 103, 4–28, 2010.
Kneuker, T., Dörr, W., Petschick, R., and Zulauf, G.: Upper crustal emplacement and deformation of granitoids inside the Uppermost Unit of the Cretan nappe stack: constraints from U–Pb zircon dating, microfabrics and paleostress analyses, Int. J. Earth Sci., 104, 351–367, 2015.
Kolodner, K., Matthews, A., Avigad, D., and Garfunkel, Z.: High temperature
Alpine metamorphism in the eastern part of the Attic-Cycladic Massive (Greece)
and its implications for early orogenesis, Israeli Geological Society Annual meeting Abstracts, p. 55, 1998.
Langosch, A., Seidel, E., Stosch, H.-G., and Okrusch, M.: Intrusive rocks in the ophiolitic mélange of Crete–Witnesses to a Late Cretaceous thermal event of enigmatic geological position, Contrib. Mineral. Petr., 139, 339–355, 2000.
Lister, G. and Forster, M.: Tectonic mode switches and the nature of orogenesis, Lithos, 113, 274–291, 2009.
Lister, G., Forster, M. A., and Rawling, T. J.: Episodicity during orogenesis, Geol. Soc. Spec. Publ., 184, 89–113, 2001.
Lister, G. S. and Baldwin, S. L.: Modelling the effect of arbitrary PTt histories on argon diffusion in minerals using the MacArgon program for the Apple Macintosh, Tectonophysics, 253, 83–109, 1996.Lister, G. S. and Forster, M. A.: White mica 40Ar/39Ar age
spectra and the timing of multiple episodes of high-P metamorphic mineral
growth in the Cycladic eclogite–blueschist belt, Syros, Aegean Sea, Greece,
J. Metamorph. Geol., 34.5, 401–421, 2016.
Lister, G. S., Banga, G., and Feenstra, A.: Metamorphic core
complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece, Geology, 12, 221–225, 1984.
Makris, J., Papoulia, J., Papanikolaou, D., and Stavrakakis, G.: Thinned continental crust below northern Evoikos Gulf, central Greece, detected from deep seismic soundings, Tectonophysics, 341, 225–236, 2001.
Makris, J. and Vees, R.: Crustal structure of the central Aegean Sea and the
islands of Evia and Crete, Greece, obtained by refractional seismic
experiments, J. Geophys.-Z. Geophys., 32.18, 42, 329–341, 1976.
Martha, S. O., Dörr, W., Gerdes, A., Petschick, R., Schastok, J., Xypolias, P., and Zulauf, G.: New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides, Int. J. Earth Sci., 105, 2031–2060, 2016.
Martha, S. O., Dörr, W., Gerdes, A., Krahl, J., Linckens, J., and Zulauf, G.: The tectonometamorphic and magmatic evolution of the Uppermost Unit in central Crete (Melambes area): constraints on a Late Cretaceous magmatic arc in the Internal Hellenides (Greece), Gondwana Res., 48, 50–71, 2017.
Massonne, H.-J. and Schreyer, W.: Phengite geobarometry based on the limiting
assemblage with K-feldspar, phlogopite, and quartz, Contrib. Mineral. Petr., 96, 212–224, 1987.
McKenzie, D.: Surface deformation, gravity anomalies and convection, Geophys. J. Int., 48, 211–238, 1977.
Palamakumbura, R. N., Robertson, A. H., and Dixon, J. E.: Geochemical, sedimentary and micropaleontological evidence for a Late Maastrichtian oceanic seamount within the Pindos ocean (Arvi Unit, S Crete, Greece), Tectonophysics, 595, 250–262, 2013.
Passchier, C. W. and Simpson, C.: Porphyroclast systems as kinematic indicators, J. Struct. Geol., 8, 831–843, 1986.
Patrick, B.: High-pressure-low-temperature metamorphism of granitic orthogneiss in the Brooks Range, northern Alaska, J. Metamorph. Geol., 13, 111–124, 1995.
Patzak, M., Okrusch, M., and Kreuzer, H.: The Akrotiri Unit on the island of Tinos, Cyclades, Greece: Witness to a lost terrane of Late Cretaceous age, Neues Jahrb. Geol. P.-A., 194, 211–252, 1994.
Pe-Piper, G. and Photiades, A.: Geochemical characteristics of the Cretaceous ophiolitic rocks of Ikaria island, Greece, Geol. Mag., 143, 417–429, 2006.
Porkoláb, K., Willingshofer, E., Sokoutis, D., Creton, I., Kostopoulos, D., and Wijbrans, J.: Cretaceous-Paleogene tectonics of the Pelagonian zone: Inferences from Skopelos island (Greece), Tectonics, 38, 1946–1973, 2019.Pourteau, A., Sudo, M., Candan, O., Lanari, P., Vidal, O., and Oberhänsli, R.: Neotethys closure history of Anatolia: insights from 40Ar–39Ar geochronology and P–T estimation in high-pressure metasedimentary rocks, J. Metamorph. Geol., 31, 585–606, 2013.
Regis, D., Rubatto, D., Darling, J., Cenki-Tok, B., Zucali, M., and Engi, M.: Multiple metamorphic stages within an eclogite-facies terrane (Sesia Zone, Western Alps) revealed by Th–U–Pb petrochronology, J. Petrol., 55, 1429–1456, 2014.
Ring, U., Thomson, S. N., and Bröcker, M.: Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece, Geol. Mag., 140, 245–252, 2003.Ring, U., Will, T., Glodny, J., Kumerics, C., Gessner, K., Thomson, S., Güngör, T., Monié, P., Okrusch, M., and Drüppel, K.: Early exhumation of high-pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey, Tectonics, 26, TC2001, 10.1029/2005TC001872, 2007.
Robertson, A. H., Trivić, B., Đerić, N., and Bucur, I. I.: Tectonic development of the Vardar ocean and its margins: Evidence from the Republic of Macedonia and Greek Macedonia, Tectonophysics, 595, 25–54, 2013.
Seidel, E., Okrusch, M., Kreuzer, H., Raschka, H., and Harre, W.: Eo-Alpine metamorphism in the uppermost unit of the Cretan nappe system—petrology and geochronology, Contributions to Mineralogy and Petrology, 57, 259–275, 1976. Seidel, E., Okrusch, M., Kreuzer, H., Raschka, H., and Harre, W.:
Eo-alpine metamorphism in the uppermost unit of the Cretan nappe system –
Petrology and geochronology, Contrib. Mineral. Petr., 76,
351–361, 1981.Sizova, E., Hauzenberger, C., Fritz, H., Faryad, S. W., and Gerya, T.: Late orogenic heating of (ultra) high pressure rocks: slab rollback vs. slab breakoff, Geosciences, 9, 499, 10.3390/geosciences9120499, 2019.
Taymaz, T., Jackson, J., and McKenzie, D.: Active tectonics of the north and central Aegean Sea, Geophys. J. Int., 106, 433–490, 1991.
Thöni, M.: Dating eclogite-facies metamorphism in the Eastern Alps–approaches, results, interpretations: a review, Miner. Petrol., 88, 123–148, 2006.
Van Hinsbergen, D. J., Torsvik, T. H., Schmid, S. M., Maţenco, L. C., Maffione, M., Vissers, R. L., Gürer, D., and Spakman, W.: Orogenic architecture of the Mediterranean region and kinematic reconstruction of its tectonic evolution since the Triassic, Gondwana Res., 81, 79–229, 2020.
Vandenberg, L. C. and Lister, G. S.: Structural analysis of basement tectonites from the Aegean metamorphic core complex of Ios, Cyclades, Greece, J. Struct. Geol., 18, 1437–1454, 1996.Velde, B.: Si+4 content of natural phengites, Contrib. Mineral. Petr., 14, 250–258, 1967.
von Blanckenburg, F. and Davies, J. H.: Slab breakoff: a model for syncollisional magmatism and tectonics in the Alps, Tectonics, 14, 120–131, 1995.Warren, C. J., Hanke, F., and Kelley, S. P.: When can muscovite
40Ar/39Ar dating constrain the timing of metamorphic exhumation?,
Chem. Geol., 291, 79–86, 2012.
Xypolias, P. and Koukouvelas, I.: Kinematic vorticity and strain rate patterns associated with ductile extrusion in the Chelmos Shear Zone (External Hellenides, Greece), Tectonophysics, 338, 59–77, 2001.
Yeung, H. S.: Timing of Central Aegean tectonic events during Tethys Ocean
Closure, MSc (adv.) thesis, Australian National University, Australia,
2019.
Zulauf, G., Kowalczyk, G., Krahl, J., Petschick, R., and Schwanz, S.: The tectonometamorphic evolution of high-pressure low-temperature metamorphic rocks of eastern Crete, Greece: constraints from microfabrics, strain, illite crystallinity and paleodifferential stress, J. Struct. Geol., 24, 1805–1828, 2002.