The stretching lineations associated with extensional shearing in the Messaria shear zone (Figure 12) show a consistent NNE-SSW orientation; the lineations plunge to the north in the northern part of the island and to the south in the southeastern part (Figure 6) and (Figure 13). Shear bands associated with the stretching lineations in the Messaria shear zone consistently indicate a top-to-the-NNE sense of shear (Figure 14). The shear bands developed during greenschist-facies metamorphism and the amphibolite-facies mineral assemblages in the Ikaria nappe were completely retrograded in the shear zone. In incipiently sheared rocks, asymmetric strain shadows around garnet contain chlorite, white mica and quartz and developed during garnet break down. Biotite is converted to chlorite in the shear bands. Staurolite breaks down to white mica and chlorite. In highly sheared rocks, relics of amphibolite-facies parageneses are lacking and the mylonite is made up of greenschist-facies mineral assemblages. In the Messaria nappe, kyanite and chloritoid in weakly sheared rocks have cracks that are filled with chlorite and quartz. Rare biotite is strongly chloritised. Mylonitic rocks usually have ribbon quartz and rotated albite porphyroclasts.
Figure 13. Messaria shear zone
(a) Map showing orientation of stretching lineations; arrowhead indicates shear sense of hangingwall. (b) Stereogram showing asymmetric folds in Messaria shear zone. The sense of asymmetry is assigned on the basis of fold vergence when viewed in the down-plunge direction. The axis and sense of asymmetry of each fold is plotted in a lower-hemisphere projection according to the convention of Cowan & Brandon (1994). The fold axes define a girdle, which parallels the orientation of the shear plane; the fold axes plot as two distinct "S" and "Z" groups. The shear direction for the shear zone can be determined from the intersection of the shear plane (average girdle of "S" and "Z" axes) and the mirror plane; the shear direction is top-to-the-NNE.
Figure 14. top-N sense of shear
Photographs illustrating the top-N sense of shear within the Ikaria shear zone.
Large parts of the I-type Raches granite in the southwestern corner of Ikaria are virtually undeformed and exhibit magmatic fabrics. Aligned igneous minerals of euhedral potassium feldspar, plagioclase, biotite and hornblende in an undeformed quartz matrix define a magmatic foliation. Thin-section analysis shows that the minerals forming the magmatic foliation do not show any signs of deformation or recrystallisation. In the upper parts of the granite, the magmatic foliation has a subhorizontal orientation. Here a weak NNE-trending magmatic lineation is defined by preferred orientation of the long axes of prismatic potassium feldspar. In its uppermost parts, the I-type granite is progressively deformed into mylonite and ultramylonite with a subhorizontal tectonic foliation associated with a very strong NNE-trending stretching lineation. The mylonitic foliation is defined by medium- to fine-grained flattened and strongly elongated quartz grains, quartz-feldspar aggregates and aligned mica. The foliation may be spaced at the millimetre scale or is extremely narrowly spaced at the micron scale. The lineation is expressed by stretched quartz-mica aggregates and recrystallised quartz tails around feldspar porphyroclasts. Quartz commonly forms polycrystalline ribbons and shows undulatory extinction, dynamic recrystallisation and grain-boundary migration. Micas are commonly recrystallised into small new grains, which define C planes that lie at a small angle to the mylonitic foliation.
The smaller S-type Karkinagrion granite occurs within the large I-type Raches granite on the southwest coast of Ikaria (Figure 4). The Karkinagrion granite has in places a weak, steeply dipping magmatic foliation, which is overprinted by a similarily oriented tectonic foliation.
The S-type Xylosirtis granite shows basically the same structural development as the I-type Raches granite and we argue that all three granites were intruded synkinematically into the Messaria shear zone.
In the Messaria shear zone numerous folds occur (Figure 15). Some of these shear-zone-related folds are strongly asymmetric on the dm to m scale. The asymmetry of the folds is consistent with top-to-the-NNE shear. The shear direction as deduced from the asymmetric folds is consistent with the orientation of the stretching lineations (Figure 13b). The ductile structures show a progressive evolution into semi-ductile and brittle structures associated with the brittle Messaria detachment. The detachment zone is made up of brecciated cataclasite in which rocks from the foot- and hangingwall are intermixed. In the Messaria nappe, marble is brecciated and jointed. Faults with offsets on the 10-100 m scale subdivide the marble into a number of tilted blocks in the hangingwall of the detachment. The sense of tilting is consistent with top-to-the-NNE displacement. Fault-slip analysis on faults in the Messaria nappe also indicates a consistent NNE-SSW-oriented extension direction (Figure 16). Throughout the upper parts of the I-type Raches granite, potassium feldspar porphyroclasts and hornblende show brittle micro-normal faults at moderate to high angles to the mylonitic foliation. These microfaults are commonly pulled apart feldspar and hornblende and growth of new quartz and biotite occurred between the displaced crystal fragments. Directly underneath the detachment fault, granite mylonite is brecciated and fractured and cataclasite and ultracataclasite occur. Very fine-grained retrograde chlorite is common in the breccia zones and feldspar is severely altered to sericite.
Figure 16. Fault-slip analysis.
Fault-slip analysis. Most small-scale faults associated with the Messaria detachment dip at angles of 30-60° to the NNE; S-dipping normal faults are distinctly steeper than N-dipping ones. NE-striking strike-slip faults are dextral and NW-striking ones are sinistral. The large arrows indicate the calculated extension directions.
Locally open folds with subhorizontal N-S-trending axes (Figure 17) fold the detachment-related stretching lineations and also appear to affect the Messaria detachment. In places, the fold hinges of these folds are decorated with ~E-W-striking extension veins and fibres in those veins trend NNE-SSW. We therefore interpret these folds as late-stage, detachment-related folds that accommodated a very limited amount of ~E-W shortening associated with NNE-directed extension.
The Fanari detachment at the contact between the Messaria and the Fanari nappe is a cataclasite zone. The cataclastic rocks have a rubbly to fragmental appearance and show numerous mesoscopic brittle faults. The fault planes are characterized by anastomosing clayey and carbonatitic gouge layers with thin (1 mm to 10 cm) zones of cataclasite, breccia and hematite-clay-coated fractured rock. Bleaching and alteration of intact rock occurs in the vicinity of faults. Weakly oriented phacoid-shaped tectonic slivers of country rock within the fault zone are on the cm to dm scale. The fault surfaces contain Riedel shears associated with lunate and crescentric structures at their intersections with the fault plane. The cataclastic fabrics record a top-to-the-NNE sense of shear. The cataclastic shear zone of the Fanari detachment is dissected by numerous small-scale faults. Fault-plane analysis shows that most of these faults have a normal sense of slip and dip at 50-70° to the south; a minor, conjugate set of normal faults dips at 60-80° to the N (Figure 16).