Temperature-time (T-t) paths for the I-type Raches granite (IK1 from Armenisits and IK2 from the south coast west of Karkinagrion), the S-type Xylosirtis granite (IK7) and the metasediments (IK6) in the footwall of the Messaria extensional fault system are slightly different (Figure 19). They indicate rapid cooling from ~700°C to <100°C within <9 Ma. Sample IK1 from the I-type Raches granite yields the most rapid cooling rate of >100°C between 9.3 Ma and 6.7 Ma followed by slow cooling thereafter. Sample IK2 depicts a similar shape of the cooling curve but cooling of this sample occurred ~2 Myr earlier than that of sample IK1. IK2 shows a fairly steady cooling from its intrusion temperature to about the apatite partial annealing zone. Sample IK7 from the S-type Xylosirtis granite had a slightly different cooling history with average cooling rates of ~80°C Myr-1 between ~10 Ma to ~5 Ma. Metasediment sample IK6 cooled somewhat slower at rates of ~70-75°C Myr-1 between ~10 Ma to ~5 Ma (Figure 19). The mean track lengths of apatite range from 14.14±0.16 μm to 14.18±0.12 μm for the I-type Raches granite, 14.43±0.21 μm to 14.51 ± 0.19 μm for the metasediments, and 14.19±0.36 μm for the S-type granite and support rapid cooling.
Figure 19. T-t diagram
T-t diagram showing cooling of metasediment sample IK6, sample IK7 from the S-type granite and sample IK1 from the I-type granite in footwall of the Messaria extensional fault system (note that intrusion age for sample IK2 of I-type Raches granite has been plotted as well). The white mica K/Ar age from sample I45/4 (Altherr et al. 1982), which is from a locality close to that of IK6 (Figure 2), has been used to extrapolate the path for IK6 to higher temperatures. Because the K/Ar white mica age of I45/4 reflects mica growth during mylonitization, a temperature range of 350-450°C has been attributed to this K/Ar age. Muscovite and biotite from the two granites show no evidence of deformation or fluid-related alteration, therefore we infer that the 40Ar/39Ar ages of these minerals reflect cooling below temperatures of 450 ± 50°C (biotite) and 500 ± 50°C (Villa 1998) (see Kumerics et al. 2005 for similar figure with temperature range for various dated minerals). The T-t path for the I-type granite is steeper for the interval between ~9-7 Ma than the paths for metasediment sample IK6 and sample IK7 from the S-type granite.
We envisage that the T-t path for the metasediments of the Ikaria nappe reflects extension-related cooling during and after greenschist-facies metamorphism and that the relatively constant cooling rate is controlled by a relatively constant rate of extensional slip. The small S-type Xylosirtis granite intruded relatively deep during extensional faulting and cooled together with the surrounding metasediments. The generally slightly faster cooling history of the large I-type Raches granite may reflect intrusion into higher crustal levels than the S-type granite. More interesting are the two cooling curves of the two samples from the Raches granite. The similar shape of both curves seems to indicate that steady cooling was controlled by relatively steady motion on the Messaria extensional fault system. Unfortunately we do not have a U-Pb age for sample IK1 from the north coast. If one projected the curve for IK2 down to higher temperatures then it seems that the intrusion age for IK1 would be ~2 Myr than that for IK2. Alternatively, IK1 has a similar intrusion age than IK2 but this part of the granite cooled relatively slowly between ~13.5-9 Ma and became only at ~9 Ma been caught up in the shear zone.
Kumerics et al. (2005) showed that the decrease of the zircon and apatite fission track and apatite (U-Th)/He ages in a direction parallel to hangingwall transport on the Messaria extensional fault system reflects the progressive southward migration of footwall exhumation. The decreasing ages are related to the lateral passage of subhorizontal isotherms at the top of the footwall. Hence, the data can be used to estimate apparent slip rates for the Messaria extensional fault system from the inverse slope of mineral age with distance in the slip direction.
Samples IK1 - IK4 from the I-type Raches granite in the western part of the island were taken from a NNE-SSW pofile parallel to the slip direction on the Messaria extensional system and yield apparent slip rates of 6.0±0.9 km Myr-1 (apatite (U-Th)/He), 8.4±6.0 km Myr-1 (apatite fission track) and 9.7±2.8 km Myr-1 (zircon fission track).
We also calculated an apparent slip rate from the K/Ar white mica ages of Altherr et al. (1982) from the Messaria shear zone. The fact that the K/Ar white mica ages of Altherr et al. (1982) consistently young in a northerly direction and are consistently slightly older than the zircon fission track ages supports the interpretation that the white mica ages date white mica recrystallisation during ductile deformation in the Messaria shear zone. This interpretation is in line with our Rb/Sr age of 12.9±3.5 Ma for sample IK99XD, which is a maximum age for mylonitisation, and our 40Ar/39Ar data of 10.8±1.1 Ma (sample IK02-4) and 10.5±2.4 Ma (sample IK02-8). The white mica ages of Altherr et al. (1982) yield an apparent slip rate of 7.3±1.0 km Myr-1, which is similar to the apparent slip rates obtained from low-temperature thermochronology.
Average apparent slip rates at the Messaria extensional fault system were ~6-9 km Myr-1. This rate would yield a displacement of ~70-100 km for the period from ~14-3 Ma.