PALEOSTRESS TRAJECTORIES AND POLYPHASE RIFTING IN ARC-BACKARC OF EASTERNPONTIDES

Yıl 1985, Cilt: 103-104 Sayı: 103-104, 1 - 15, 01.08.1985

Osman Bektaş

Öz

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.

Anahtar Kelimeler

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Kaynakça

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