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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: On the Origins of Upthrusts
By
Esso Production Research Company
Upthrusts can originate from strike-slip faulting and from differential rotation of
asymmetric blocks. Upthrusts are known along several major strike-slip faults and
they can also be produced by purely lateral motion in clay experiments. Those of
strike-slip origin are a class of upthrust in which compression plays an important role.
The asymmetric-block upthrusts of the Rocky Mountains foreland type do not show
the effects of strong compression; nor do they usually show the expected geometric
pattern of strike-slip; nor has strike-slip been demonstrated for them by piercing points.
They may evolve by block rotation from both normal faults and previously formed
strike-slip
faults at depth.
Field observations suggest that upthrusts can become normal faults at depth. This
evidence has prompted clay-model experiments in which an upthrust was created in
clay
above a preexisting normal fault separating two wooden blocks as the blocks were differentially rotated. A model of differential block rotation satisfies fundamental observations
about the asymmetry and the amount of compression associated with asymmetric block
upthrusts. Their asymmetry can be created, or inherited, or both, from the deeper
normal-fault geometry of tilted, rotated blocks, where the normal-fault face constitutes
an asymmetric, steep flank, and the gentle flank is the unfaulted, tilted side;
asymmetric basins are similarly explained, essentially as half-grabens. Compression
from differential rotation of blocks is manifest mainly at the junction of the blocks.
However, since differential rotation puts entire segments of basins under compression
which effectively acts as a body force, bedding-plane thrusts can occur basinward of
the upthrust zone in the sedimentary sequence where shear strength is exceeded by
compressional stress. End_of_Record - Last_Page 28--------