DEFINITION OF PULL-APART BASIN

An elongate depression that develops

along extensional steps on strike-slip faults. Pull-apart basins

are features that develop in transtensional regions, in which

the principal stresses are compressional, but some areas within

the region are under extension due to the obliquity of the

major stress direction with respect to plane of failure. This

results in extension of the crust along releasing bends, leading

to a break in the crust and the formation of basins. Some

pull-apart basins show several progressive stages in their formation.

Others initiate along a fracture, progress into lazy Z

or S shapes, and finally progress into a basin that ranges in

length to width ratio from 2:1 to 10:1. These types of basins

are characterized by steep sides on major fault boundaries

with normal faults developing on their shorter sides. Continuous

movement along the major faults tend to offset deposits

from their source inlet to the basin. These basins are characterized

by rapid deposition and rapid facies changes along or

across the width of the basin and gradual facies changes

along the longest axis of the basin. Pull-apart basin deposits

are typically made mostly of coarse fanglomerate, conglomerate,

sandstone, shales, and shallow water limestones and

evaporites. Bimodal volcanics and volcanic sediments are also

found interbedded within the basin deposits. These bimodal

volcanics are typical of those found in rift settings, but here

they are in a transtensional regime. Transcurrent faults can

penetrate down deep into the crust reaching the upper mantle

and providing a conduit for magma.

See also PLATE TECTONICS; TRANSFORM PLATE MARGIN

PROCESSES.

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