The Marmara Region of northwest Turkey covers the Marmara Sea Basin and the surrounding lands. In the region morphologically different areas may be distinguished. The Marmara Sea Basin is a structural depression. The land to the north is known as the Thrace-Kocaeli Peneplain a flatland elevated above a horst. This may be referred to as the Istanbul Horst bounded by two strike-slip fault zones which define the structural boundaries of two sea basins