My PhD Thesis was completed and submitted on July 4th 2001.

TITLE:  Episodicity during Orogenesis

This PhD project involved Structural Geology and Argon Geochronology. The aim of the project was to understand the deformational events and undertake geochronological analysis of two distinct collisional settings, the blueschist belt of the Aegean in Greece and the Otago Schist New Zealand, and compare the results.

The following are the conclusions from this work:.

Episodes of both deformation and metamorphism in back-arc collisional settings, such as the Aegean and the Otago Schist, appear to be an intrinsic geological phenomenon. Deformational and metamorphic episodes occur over relatively short periods of time, with discrete but regional effects. Episodes of deformation can be associated with the changing character of the adjacent subducting slabs, related to roll-back, as well as to effects external to the immediate collisional setting (e.g., the effects of the breakup of Gondwana on the Otago Schist).

Pervasive regional-scale metamorphism occurs in these tectonic settings, as has been observed in the blueschist belt of the Aegean, or in respect to the pervasive greenschist conditions of the Otago Schist. However, as the orogeny develops, subsequent metamorphic episodes overprint the older event(s) and yet do not necessarily obliterate them. Pristine regions of a single metamorphic facies assemblage (e.g., eclogitic assemblages) are retained, although complex overprinting of different metamorphic facies assemblages is characteristic. This is observed in the Cyclades where different combinations of overprinting of HP metamorphic events (M1A, M1B, M1C, M1D) affect individual tectonic slices differently (in the period between ~65-30 Ma). The same pattern is observed for subsequent greenschist-facies metamorphic events (M2A, M2B), in the period ~25-12 Ma. Similarly, in the Otago Schist, three separate episodes of static recrystallization, metamorphism, and grain growth have been observed, in the period ~125-100 Ma. These are developed with different intensity depending on structural location relative to different sets of shear zones.

The variation in preservation and overprinting of different metamorphic episodes can be explained if the episodes are short in duration and if note is taken of possible variation in structural depth dependent on the effects of different ductile shear zones and/or detachment faults. Part of a region that has undergone eclogite facies metamorphism can be exhumed by ductile shear zones and consequently be overprinted by metamorphism that reflects PT conditions that are quite distinct and different to those that take place in the remainder of the region. If a subsequent exhumation event exposes both parts of the region these different and contrasting PT histories will be exposed to view. This can occur during both extension and shortening, or both, and can occur over and over again.

In addition it appears that metamorphism can be caused by deformation and in fact deformational and metamorphic episodes can be intrinsically linked. In the Otago Schist, at and adjacent to the Caples/Torlesse boundary, an increase in metamorphic conditions occurs specifically within and immediately below the large-scale (extensional?) shear zones. This variation in metamorphic evolution can be explained if thermal episodes are synchronous with, but outlast the shearing events. Shear heating and/or effects associated with fluid ingress associated with the operation of these large-scale shear zones could be the cause of this localised increase in temperature.

In the Otago Schist the 40Ar/39Ar apparent age of micas in shear zones is retained relatively unaffected by younger recrystallization associated with subsequent thermal episodes. These episodes must have been short-lived as they represent temperatures hot enough to reset the argon system over a prolonged period, and such resetting in general has not taken place.

Absolute dating (i.e., 40Ar/39Ar geochronology) has aided structural analysis in the unravelling of the tightly constrained metamorphic and deformational episodes that have occurred in both the Aegean and the Otago Schist. Both these region have resided within the Argon Partial Retention Zone (with relatively low temperatures). This enabled the retention of older metamorphic assemblages despite new mineral growth, and the retention of older reservoirs of 40Ar within K-bearing minerals. Frequently measured ages (FMAs) produced from such samples represent specific metamorphic and deformational episodes, and application of the method of asymptotes and limits has produced a sequence of geologically significant ages that can be correlated over these regions. Ages of the different metamorphic and deformational episodes can be constrained to specific time periods, and these ages correlate with the relative chronology determined by fabric and microstructural analysis.

40Ar/39Ar apparent age spectra do not represent cooling ages in the Otago Schist or the Aegean, rather the ages represent recrystallization ages associated with deformational and/or metamorphic episodes. Prolonged periods of either deformation or metamorphism have not been detected, in either the structural analysis or the 40Ar/39Ar geochronology. Where sequence of events occur or recur they often represent a change in tectonic regime from extension to shortening or vice versa. Where deformation events appear to occur over longer period with the same kinematic and tectonic regime, there are distinct time breaks suggesting episodes of deformation occur, rather than an ongoing progressive process. This may have happened for example in the Northburn shear zone in the Otago Schist where N-S directed shearing occurred in the period ~112-109 Ma and then again at ~91-86 Ma.

It is suggested that the episodic nature of the deformational and metamorphic events observed in the back-arc settings of the Aegean and the Otago Schist are not unique cases. Different back-arc setting will have their own unique geological features as do the Aegean and the Otago Schist, but the fundamental characteristic of episodicity as described may well be intrinsic to this tectonic setting.

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