Development of Pull-Apart Basins

Abstract

A comparative study of well mapped active and ancient pull-apart basins suggests a qualitative model for their continuous development. Pull-aparts evolve from incipient to extremely developed basins through a sequence of closely related states. New and compiled map data from several areas, including the northern Caribbean and Turkey, suggests the following stages in pull-apart development: (1) in rigid, intracontinental strike-slip plate boundary zones, larger pull-aparts nucleate at releasing fault bends along segments of the principal displacement strike-slip fault zone which are oblique to the theoretical interplate slip lines; (2) initial opening across releasing fault bends produces spindle-shaped basins defined and often bisected by oblique-slip faults connecting the discontinuous ends of the strike-slip faults; (3) increased strike-slip offset produces basin shapes which we colloquially call "lazy S"-shape for basins between sinistral faults and "lazy Z"-shape for basins between dextral faults; (4) rhomboidal pull-aparts or "rhomb grabens" result from lengthening of an S or Z-shaped basin with increased strike-slip offset and characteristically contain two or more sub-circular deeps within the basin floor; and (5) prolonged strike-slip over tens of millions of years can produce long narrow troughs floored by oceanic crust created at an orthogonal short spreading ridge; basin width does not increase significantly and remains fixed by the width of the releasing bend. Most pull-aparts have low length to width ratios, and this is a consequence of their short lives in rapidly changing strike-slip zones.