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Timing of the closure of the ancient Tethys Ocean that separated Eurasia and Gondwana

M.A. Forster

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Our understanding of Plate Tectonics has continued to advance since the late 1960s, in particular with knowledge of what is happening beneath the surface. Data from seismology and tomography provides 3D views 100 - 1000 km below the Earths’ surface.  But to understand plate tectonic movements and processes we also need to understand the timing of major events. In this aspect what the rocks have to tell us is of vital importance, although we must learn how to extrapolate information gleaned on the micro-scale to constrain different hypotheses as to how large-scale tectonic movements take place. This is the intent of the “Closing Tethys” project recently funded by the Australian Research Council.

The best places to work to collect data that time critical events during the closure of the ancient Tethys Ocean are in the exhumed roots of mountain belts that mark the effects of collision. Of particular interest are deep cutting suture zones with ductile deformation and rocks that have been at pressure and temperature of the deep crust or mantle rocks have been brought to the surface, and thus in the “accessible crust” provide a long history of information.  These key locations are “orogenic listening posts” that sensitively record the effects of events during closure of Tethys.  We have chosen five such, spread over a distance of ~12,000km along the northern boundary of this ancient ocean.

Dating sequences of events that record effects related to the movement of the plates can be done by dating the ductile fabrics that form as the plate moves and deforms. We can sample critical locations, and interrogate suitable rocks as to when events took place in locations such as the high-pressure belts of Greece and Turkey, the deep sutures of the Himalaya or the early high-pressure rocks beneath obducted sheets in SE Asia. In this aspect the argon isotopic system is the key geochronological tool at our disposal, providing information that allows constraints as to the nature and origin of the geodynamic processes involved in forming and/or modifying specific fabrics, as well as absolute timing and duration constraints in the relative chronology of events in a specific region.

Argon geochronology is the only geochronometer that allows direct dating of common rock forming minerals in fabrics. Therefore it has been the geochronometer of choice in this work, and it will continue to be used to date events in key locations along the Tethyan belt. There is an additional advantage in using the argon geochronometer, since we can engage in geospeedometry to determine how long individual rocks endured peak conditions, e.g., we can tell how long a particular ductile shear zone operated by dating different elements of its microstructure, or we can place a limit on the maximum duration of heating during individual metamorphic events.

Figure 2. The Himalaya orogeny making a collisional zone between Eurasia and India> The ancient 'Tethys Ocean' now lies fragmented along this belt.