BROAD SHEAR ZONES AND NARROW STRIKE-SLIP FAULTS IN OROGENS AND THEIR ROLE IN FORMING THE OROGENIC ARCHITECTURE: THE NORTH ANATOLIAN FAULT AS AN ACTIVE EXAMPLE

The earth's crust is generally more pervasively faulted than is commonly believed. Most faults are of normal type and almost as many are thrusts. Strike-slip faults occur not as frequently as the others (except on small scales), but when they do in a big way, they influence the orogenic architecture in manners very different from the other two types. The reason for this is that there is no limit to how much displacement they can have in contrast to the other two. Strike slip faults begin as broad shear zones. The location and the breadth of such zones is determined by the rock composition of the curst and the geometric distribution of rock types. Young shear zones may pinch an swell along the strike and within them strain geometries evolve creating complex and overprinting structure patterns they give the misleading impression that the stress regimes may have changed while they evolved. As the evolution of the shear zone continues one or more shear-zone-parallel through-going strike-slip faults consisting of families of closely packed fault segments localise while other structures cease their activity. In most cases one through-going strike-slip fault eventually dominates and the displacement along the shear zone may concentrate itself just to it. Before this stage is achieved shorter strike-slip faults may join creating releasing and/or restraining bends superimposing a new fabric on the pre-existing orogenic fabric and create new depocentres lying unconformably above the older structures. Such depocentres may themselves fall prey to strike-slip faulting and get deformed in their own turn. Where the main fault trends deviate from the regional displacement vectors, zones of transtension and/or transpression form that have extremely complex fabric patterns that give rise to what Dewey has termed 'spoof orogeny'. All the structures and stages of evolution just mentioned can be seen in the history of the North Anatolian Fault. We here present a history and structural synthesis of this fault with emphasis on its western one third with a view to illustrating the rôle of shear zones and strike-slip fault systems in fossil orogens by describing one that is now evolving before our eyes and about which we have a very large set of multidisciplinary submarine and subaerial geological as well as geophysical data.