Itinerary 1: Blueschists and carpholite-schists northwest of Katapola
The itinerary will take you through the high-pressure rocks northwest of Katapola (Figure 1), focusing on the low-angle contact between two different high-pressure units and the F3 folding of the overlying marble sequence. To reach the area, start walking along the coastline through the northern part of Katapola towards the cemetery. A map showing the itinerary and locations of the excursion stops is presented in (Figure 2).
Stop 1.1. Flysch (N 36º49’55.1”, E 25º51’41.0”)
Flysch is outcropping on the road from Katapola towards the cementery. The flysch consists of medium-grained sandstone and interlayered mudstone. Bedding and cleavage are developed but usually any kind of layering is highly disrupted. In this locality, the occurrence of the flysch is in the hangingwall relative to a detachment fault (see Stop 1.5).
Approximately 100 m from the cemetery, we collected a sandstone sample for fission-track dating (sample AMO 05-27). This sample contained zircon, but not apatite. It yielded an Early Cretaceous zircon fission-track age of 134±9 Ma. Another fission-track sample from the hanging wall of the Amorgos detachment was collected from flysch sandstone along the road to Chora and yielded a Late Triassic age of 219±16 Ma. Both fission-track ages are much older than the Eocene (?) depositional age of the flysch, indicating that metamorphic temperatures in the flysch were not sufficiently high to completely anneal the fission tracks in zircon. This in turn suggest that metamorphic temperatures remained below 300°C.
Stop 1.2. Conglomerate (N 36º50’14.1”, E 25º51’30.3”)
Continue walking on the road until it ends (near a small chapel) and becomes a footpath. Approximately 100 m after the beginning of the footpath, you will reach a gully and a small water spring. The rocks in this locality consist of strongly deformed conglomerates alternating with micaschists. They have been mapped as part of the carpholite-bearing unit (Figure 2), although they could possibly represent a sheared section of the flysch conglomerate. The conglomerate is characterised by up to 5 cm large quartz pebbles. Structures are associated with high-angle shear bands that indicate top-to-the-SE sense of movement (Figure 3), and crenulations that define an axial plane parallel to the shear bands.
Figure 3. High-angle shear bands in a conglomerate (Stop 1.1) showing top-to-the-SE sense of shear. Sample AMO 05-1 from this conglomerate yielded a zircon fission-track age of 161±15 Ma.
Figure 4. Metabasite blueschist-facies rock (dark green) exposed below a low-angle detachment contact (Stop 1.2).
Figure 5. Carpholite-bearing rocks from the area around Stop 1.2.
(a) Strongly sheared micaschist; (b) Stretching lineation indicated by fibrous carpholite grains.
Stop 1.3. Blueschist-facies rocks (N 36º50’18.7”, E 25º51’21.6”)
The rocks around this stop represent some of the most interesting metamorphic assemblages in Amorgos. One of these is found in a dark green metabasitic rock, which is exposed in a small outcrop on the footpath and immediately below it (Figure 4). This rock has a penetrative subhorizontal foliation on which a shallowly NW-plunging stretching lineation occur. The stretching lineation is expressed by aligned chlorite flakes and elongated albite.
Thin section examination revealed that the metabasite consists of blue-amphibole, garnet and clinopyroxene, indicating peak P-T conditions of >13 kbar and 500-600°C (Rosenbaum et al., 2007). The blue amphiboles are typically magnesio-riebeckite with rims of green amphiboles (tschermakite). Garnet occurs as porphyroblasts, strongly altered to epidote and chlorite and rich in actinolite, clinopyroxene, quartz and apatite inclusions. Accessory calcite and sphene have also been recognised in thin sections. The overall preservation of the blueschist assemblage is relatively poor due to the effect of strong greenschist-facies overprint.
Metabasite sample AMO 04-16 yielded a few apatite grains, which provide an apatite fission-track age of 18.3±3.2 Ma. This age is based on only 12 dated grains but nevertheless indicates that detachment-fault-related cooling occurred in the late Early Miocene (Burdigalian).
Rocks immediately above the metabasitic block are strongly deformed schists (Figure 5a). The schists consist of quartz, phengite, paragonite and fibrous Fe-Mg-carpholite grains. The carpholite fibers are spectacular and form distinct cm- to dm-long carpholite-quartz-phengite aggregates that define a NW-trending stretching lineation (Figure 5b). Peak metamorphic conditions of 300-450°C at 10-14 kbar have been estimated for this assemblage (Rosenbaum et al., 2007), indicating that this assemblage was formed in a different thermal regime compared with the adjacent blueschist-facies metabasitic rock. Therefore, the contact between the two rocks is interpreted as a tectonic contact.
The contact between the two high-pressure rocks is a strongly sheared low angle detachment fault (Figure 4). It is characterised by a 1-2 m thick zone of semi-ductile to brittle structures that grade structurally upwards into a gouge layer. A top-to-NW sense of shear is inferred based on kinematic indicators.
Stop 1.4. Carpholite-bearing meta-conglomerate (N 36º50’19.9”, E 25º51’21.2”)
Keep walking for 70-80 m up the track until reaching a platform of a meta-conglomerate rock. This rock has a purple to dark green colour, which is distinctly different from the abundant conglomerates of the flysch unit. The pebbles are derived from milky white vein quartzites, marbles and schists. Fibrous carpholite can be recognised in the matrix. Because the pebbles of the flysch conglomerates contain a much larger variety of clast lithologies, including numerous dm-large marble pebbles, we strongly believe that this conglomerate does not belong to the flysch unit.
Fission-track sample AMO 05-22 from this outcrop yielded a Middle Jurassic zircon fission-track age of 167±16 Ma, which is similar to the zircon fission-track age from the carpholite-bearing conglomerate at Stop 1.1. These older fission-track ages have nor been reset during the Tertiary orogenic history of this unit. This suggests that metamorphic temperatures remained below 300°C, thus at the very lower end of the estimated peak PT conditions reported above.
Stop 1.5. The detachment (N 36º50’26.8”, E 25º51’42.1”)
Leave the track before its sharp turn westward and walk northeastward along the contact between high-pressure albite-micaschists and flysch conglomerates. Keep walking until reaching a relatively good exposure of the detachment contact in a little rock cliff (Figure 6). The cliff consists of strongly sheared carbonate-bearing phyllites. The penetrative foliation developed under lower greenschist-facies conditions, but abundant gouge layers developing in the foliation planes attest to ongoing shear along the foliation planes in the brittle crust. There is evidence for both to top-to-the-SE and top-to-the-NW sense of movements. Based on structural evidence elsewhere in the island, it seems that the top-to-the-NW sense of shear is associated with the actual movement along the detachment, whereas the top-to-the-SE sense of shearing is inherited from an earlier deformation (Rosenbaum et al., 2007).
Stop 1.6. Refolded F1 folds in the flysch (N 36º50’33.2”, E 25º51’44.3”)
Approximately 200 m northward from the previous stop, you can find excellent examples of refolded structures in the flysch (Figure 7). These structures are type 3 interference patterns (Ramsay, 1967) associated with refolding of F1 isoclinal folds around F2 folds. The F2 folds are asymmetric Z shaped (down plunge) with fold hinges that shallowly plunge towards the east.
Stop 1.7 Dolomite (N 36º50’17.8”, E 25º51’05.4”)
Walk back to the curve in the main track north of stop 3, and continue westward for approximately 250 m. The rock here consists of yellow dolomite layers alternating with purple marly shales and meta-claystones (Figure 8). The meso-scale folds in the dolomite are interpreted as late F3 folds.
Stop 1.8. Albite micaschist (N 36º50’17.2”, E 25º51’00.9”)
From Stop 6, head southwestward to the crest of the peninsula. You will notice that the dip of the dominant foliation changes due to the effect of F3 folding. The foliation is nearly vertical in Stop 7, where a narrow band of albite-mica schists is exposed. This rock supposedly represents a lower structural level (part of the basal conglomerate unit) which reappears here as a result of folding and faulting. Adjacent to the micaschists, you can find a small outcrop of the meta-conglomerate with its characteristic milky white quartz veins.
Stop 1.9. Large F3 fold (N 36º50’14.6”, E 25º50’26.8”)
The last stop in this itinerary is located near the small chapel of Profitis Ilias in the westernmost part of the peninsula. In this locality, there is an excellent exposure of the fault contact between marbles in the hanging wall and micaschists and meta-conglomerates in the footwall (Figure 9a). The overlying marble is folded showing a subhorizontal fold hinge and an axial plane that is shallowly dipping towards the east.
Figure 9. Large F3 fold in the westernmost part of the peninsula (Stop 1.8).
Note the excellent exposure of the fault contact between footwall schists and conglomerates and hanging wall marbles.