Testing alternative hypotheses for the Early Mesozoic evolution of Tethys in Western Turkey
Alastair Robertson1, Timur Ustaomer2, Theo Andrew1, Alan Collins3, Elizabeth Pickett4 and J.E. Dixon1
1 Geology and Geophysics, Edinburgh Uni., Edinburgh EH9 3JW; 2 Dept. Geological Engineering, Istanbul Uni., 3 Curtin Uni., Perth, Australia, 4 British Geological Survey, Edinburgh
Alastair.Robertson@glg.ed.ac.uk
To reconstruct any particular orogen for a particular time slice it is necessary to integrate a broad data set, including structural, sedimentary, igneous, metamorphic, biostratigraphical and geophysical information.
A case in point is the current debate concerning the Early Mesozoic tectonic evolution of the Tethyan orogen in Western Turkey, specifically during early Mesozoic time. There are two main competing hypotheses. In the first the Tethys (i.e. Palaeotethys) existed in Late Palaeozoic-Early Mesozoic as a wide ocean separating Gondwana (i.e. Anatolia) to the south from Eurasia in the north (i.e. Pontides). This ocean completely closed during a Mid-Late Triassic Cimmerian orogeny of Himalayan/Tibetan type. The suture then rifted in the Early Jurassic time to open a new Mesozoic ocean basin system (Neotethys) to the south. In the alternative hypothesis an early Mesozoic ocean (Palaeotethys) did not close in Triassic time but instead remained continuously open and evolved into the Mesozoic Neotethys without any fundamental break.
To test these alternatives we utilise evidence from the Pontides (northern margin) and Anatolides/Taurides (southern margin), largely from Edinburgh-based PhD studies.
For the northward margin (e.g. central Pontides) we see evidence of northward subduction/accretion, HP/LT metamorphism, arc volcanism and ophiolite genesis in Triassic time. This culminated in deformation and regional metamorphism prior to a Late Jurassic shallow-marine transgression.
For the southern margin (Anatolides/Taurides) we find that major nappe systems (Lycian and Beysehir Hoyran-Hadim nappes) were rooted to the north of the Tauride continental units (e.g. Menderes and Bolkar Dag massifs). These nappes include a record of Early Triassic rifting and Mid-Late Triassic continental break up to create Neotethys (i.e. northerly Neotethys). We see no evidence of a regional continental collision event in the Mid-Late Triassic. The available evidence thus supports the second (open-ocean) hypothesis.
Our present view is that Triassic Palaeotethys was dominantly subducted northwards under the Eurasian margin, associated with accretion (e.g. Domuz Dag melange), marginal arc volcanism (Cangaldag arc), and SSZ-type spreading (e.g. Elekdag ophiolite). Continental fragment(s) associated with hot-spot-type volcanism were rifted from the southern margin opening a Triassic Neotethys along the southern margin of the Tauride continental block(s). These "Cimmerian" fragments drifted across the Tethys (e.g. Cal unit of Karakaya Complex) until they were accreted to the Eurasian margin prior to Late Jurassic time. The Tethys remained continuously open but evolved into Neotethys, comparable to processes active in the recent SW Pacific. Tethyan collision was later set in train by Mid-Late Cretaceous northward subduction, culminating in Early Tertiary diachronous collision.