Abstract
Determining the tectonic evolution and thermal structure of a tectonic unit that experiences a subduction-related pressure temperature (P-T) loop is challenging. Within a single unit, P-T conditions can vary from top to bottom which can only be revealed by detailed petrological work. We present micropetrological data from the middle section of the Cycladic Blueschist Unit (CBU) in Naxos, Greece, which indicates a different P-T loop than that for the top of the sequence. Using Zr-in-rutile and Ti-in-biotite thermometry coupled with quartz-in-garnet elastic barometry and phase equilibrium thermodynamic modeling, we identify a prograde path from 15.4 ± 0.8 kbar to 19.9 ± 0.6 kbar and from 496 ± 16 °C to 572 ± 7 °C (2σ uncertainty), equilibration during decompression at 8.3 ± 1.5 kbar and 519 ± 12 °C followed by near-isobaric heating to 9.2 ± 0.8 kbar and 550 ± 10 °C (or even 584 ± 19 °C), and a final greenschist-facies equilibration stage at 3.8 ± 0.3 kbar and 520 ± 4 °C. We compare these P-T estimates with published data from the top and also the bottom of the CBU section and find that the bottom half of the CBU on Naxos records higher peak high-pressure (HP) of about 4 kbar than the top of the unit, defining the thickness of the CBU section on Naxos to about 15 km in the Eocene. We determine that crustal thickening of up to ~15% occurs in the upper half of the CBU section during exhumation of the HP rocks in an extrusion wedge in a convergence setting. At about 30 Ma, the bottom half of the CBU was finally thrust onto the radiogenic Cycladic basement. Subsequently this bottom half of the CBU section underwent isobaric heating of 9–96 °C between c. 32–28 and 23–21 Ma. Isobaric heating occurred below the upper CBU section that thickened during decompression and commenced when HP metamorphism in the Cyclades ended. This suggests that thermal relaxation following tectonic accretion in the Cyclades controlled heating of the evolving Cycladic orogen during a tectonically quiescent period before lithospheric extension commenced by 23–20.5 Ma.
Highlights
Exhumation of high-pressure (HP) rocks in subduction-zone settings often occurs in extrusion wedges (Chatzaras et al, 2006; Gessner et al, 2001; Glodny et al, 2008; Ring and Glodny, 2010; Xypolias et al, 2003)
We show that the upper half of the Cycladic Blueschist Unit (CBU) wedge, largly above the biotite isograd, below the bounding upper normal fault experienced continuous modest cooling during decompression between c. 40 ± 2 and 32 ± 2 Ma and thickened during exhumation while the CBU was thrust onto the Cycladic basement
Our findings show that isobaric heating commenced immediately after the CBU was thrust onto the radiogenic Cycladic basement
Summary
Exhumation of high-pressure (HP) rocks in subduction-zone settings often occurs in extrusion wedges (Chatzaras et al, 2006; Gessner et al, 2001; Glodny et al, 2008; Ring and Glodny, 2010; Xypolias et al, 2003). An extrusion wedge is defined by two main fault structures working in concert during overall horizontal lithospheric shortening: a thrust fault at the base of the wedge and a normal fault at its top, the latter of which facilitates exhumation. There could be a series of normal faults at the top of the wedge thinning the wedge. Note that any thickening of an exhuming wedge needs to be less than the thinning at the top of the wedge along the upper normal fault(s). In subduction zones the exhumation in extrusion wedges often occurs rapidly at rates close to the rate of subduction (Gerya et al, 2002; Ring and Reischmann, 2002; Rubatto and Hermann, 2001). The typical time frame of 20–30 Ma needed for thermal relaxation (Glazner and Bartley, 1985) does not play a significant role (Peillod et al, 2017) and the extrusion wedge undergoes isothermal decompression or slightly cools during exhumation
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