Tear faults can be be defined as relatively small-scale, local strike-slip faults that are associated with other structures such as folds, thrust faults and normal faults (Twiss and Moore 1992). In a thrust belt the tear faults strike normal (perpendicular) to the thrust front and fold axis. As the thrust sheet propagates into the foreland the tear faults start to segment the thrust sheets and eventually act as lateral boundaries that seperate the thrust units (see diagram below). The tear faults may be a result of drastic lateral changes when the thrust front advances or by differential shortening of the thrustal sheet. For example as one\ part of the thrust sheet is shortened by faulting and the other is shortened by folding, the displacement is taken up by the tear fault (see diagram). The Jacksboro fault in the Appalachians is a good example of a tear fault that segments the Pine Mountain Thrust Belt (Twiss and Moores 1992).
Reference: Twiss, Robert J. and Moores, Eldridge M.; Structural Geology W. H. Freeman and Company, 1992, pp 103 and 113.
Above can be seen a small tear fault that linkgs the backthrust with the irregular thrust front.