PALEOSTRESS TRAJECTORIES AND POLYPHASE RIFTING IN ARC-BACKARC OF EASTERNPONTIDES

Abstract

In the modern convergent plate margins geological and geophysical evidences imply that maximum horizontal stresses (sHmax) over the overriding plate are transmitted from plate boundry to the backarc region. This causes compressive regime in the plate boundry and extensive regime in the backarc or inner part of overriding plate. Depending on the age and properties of downgoing plate and relative motion of the overriding plate maximum compressive stresses (s) are transmitted as s2 (sl> s2> s3) in the consuming direction or parallel to the arc from trench zone to backarc region. Though strike slip motions are dominant in the arc, they are associated to extensional and compressional regions. Geological data from Eastern Pontides, especially southern part of arc seem to show that tecto- nics is predominantly extensional and several short lived compressional phases break up this extentional regime during Mesozoic same as in the Agean and Japan arcs. First extensional regime started in Lias or Pre-Lias and ended in Malm-Late Lower Cretaceous. In this period many ensialic intra-arcbasins to the north and ensimatic backarc basins with axial through sea floor spreading (Malm-Lower Cretaceous ophiolite) to the south had been developed (Mariana type subduction). Short lived compressional phase between Late Lower Cretaceous and Early Upper Cretaceous detracted these basins (Chilean type subduction). Under a new extensional regime Eastern Pontian are and backarc rifted again and new axial sea floor spreading occurred to the south to form Upper Cretaceous ophiolites. The formation of Krukko type polymetalic ore deposits along the Black Sea coast correspond to this stage (intra-arc rifting). Intra- arc and backarc basins closed again by following compressive stresses between Late Upper Cretaceous and Early Eocene. Except for sea floor spreading polyphase rifting should have been in the same way during Cenozoic time. In addition to diverse folding axes and opposite direction thrusting may imply that strike slip motion may be associated to compressional and extensional regime in Eastern Pontides. As a result except very short lived compressive stresses southern part of Pontides is the extensional region or extensive stresses increase from north to south. Such a result indicates that southern part of Pontides was the backarc region and it is in favor of southward subduction during Meso- zoic and Cenozoic time.