Well-exposed ancient pull-apart basin deposits have the following characteristics in common: (1) great stratigraphic thickness relative to basin size, (2) high rate of sedimentation, (3) asymmetry of sediment thickness and facies pattern, (4) organization of the facies into marginal fault-bounded fanglomerates and central flood-basin and lacustrine deposits, and (5) textural cycles that reflect tectonic activity and evolution. The relationship between basin deposits and boundary faults is usually well defined. However, many pull-apart basin deposits are deformed and their boundary faults are usually no longer active. Understanding of sedimentation in ancient pull-apart basins suffers from a limited knowledge about the original depositional and structural setting. In an at empt to develop a more comprehensive understanding of sedimentation in pull-apart basins, we examine the fault and depositional systems of two modern, active pull-apart basins in eastern Turkey.

The Erzincan pull-apart basin overlies the North Anatolian fault, a right-lateral, 1,200-km (745 mi) long intracontinental transform fault. It is 50 by 13 km (31 by 8 mi) and rhomboidal. Boundary strike-slip faults define sharp, steep, and relatively undissected mountain fronts. These active faults are characterized by an overlap of 32 km (20 mi) and a separation of 13 km (8 mi). Sedimentation within the basin is controlled by: (1) an axial fluvial system (the upper Euphrates River) associated with a broad flood plain/salt marsh, and (2) alluvial fans that prograde transversely from the boundary mountain fronts. Small calcalkaline volcanic cones intrude and overlie these sediments along subsidiary strike-slip faults within the basin. Aluvial fans are well-developed along the northern ountain front. They are steep, composed of fluvial and debris flow facies, and prograde over fluvial plain/salt marsh deposits. Along the southern mountain front alluvial fans have coalesced into a gently sloping alluvial apron composed of fluvial facies. These features suggest that the northern fault margin has probably been more recently active than the southern fault margin.

The Lake Hazar pull-apart basin overlies the East Anatolian fault, a left-lateral, 450-km (280 mi) long intracontinental transform fault. It is 25 by 7 km (16 by 4 mi), rhomboidal, and forms an asymmetric half-graben in cross section with its deepest part to the south. Boundary strike-slip faults form sharp, steep mountain fronts and are characterized by a separation of 3 km (1.9 mi) and an overlap of about 5 km (3.1 mi). Sedimentation within the basin is controlled by: (1) an axial fluvial system which enters the basin longitudinally and forms a large (5.1 km² [2 mi²]), low-gradient (1.1°) fan delta which is composed of interbedded fluvial and lacustrine facies, and (2) lateral fluvial systems that enter the basin transversely and form small (0.03 to 0.23 k ² [0.01 to 0.09 mi²]), steep (2.5° to 8.5°) fan deltas composed of interbedded fluvial and debris flow facies.

The Erzincan and Lake Hazar pull-apart basins present an "instantaneous" structural and depositional picture of pull-apart basin evolution and provide a basis of comparison with ancient pull-apart basin deposits.

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