In a reverse or thrust fault, the hanging wall has moved up relative to the footwall. The distinction between a reverse fault and a thrust fault is that a reverse fault has a steeper dip, greater than 30 degrees. Reverse and thrust faults develop in sectors of the crust that are experiencing compression. In this regard, a convergent plate boundary is a zone of main reverse and thrust faults. Subduction zones; consequently, are occasionally stated as mega-thrust faults (Pollard et al., 2005). In this understanding, reverse and thrust faults similarly arise in other surroundings where the crust is befalling compressed; for instance, the Transverse Mountain Ranges, just north of Los Angeles (Fig. 2.13).
The geometry of thrust systems
Thrusts are reverse faults so that the hanging wall moves up relative to the footwall. As a consequence, if thrusting affects horizontally bedded strata, deeper (older) rocks are carried onto shallower (younger) ones (Fig. 9.4A). However, thrusts rarely cut smoothly upsection. Rather, they tend to have segments that are subhorizontal (termed flats), connected by steeper segments (called ramps) that collectively form staircase trajectories through strata. As Rich noted, movement up thrust staircase trajectories create folds in the hanging wall, simply as a geometric response to the fault shape. These are fault-bend folds (Fig. 9.4B). Displacement on the thrust surface is passed beneath the fold. However, as thrusts lose displacement up-dip along a ramp, the resultant structure is termed a fault-propagation fold (Fig. 9.4C). These structures have distinctly steeper forelimbs than fault-bend folds. The up-dip termination of thrusts faults are called tips. Folds formed above a basal slip surface, without thrusts cutting their forelimbs, are termed detachment folds (Fig. 9.4D). In effect, these form in response to displacement gradients on thrust flats, rather than ramps as for fault-propagation folds.
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Thrusts are reverse faults so that the hanging wall moves up relative to the footwall. As a consequence, if thrusting affects horizontally bedded strata, deeper (older) rocks are carried onto shallower (younger) ones (Fig. 9.4A). However, thrusts rarely cut smoothly upsection. Rather, they tend to have segments that are subhorizontal (termed flats), connected by steeper segments (called ramps) that collectively form staircase trajectories through strata. As Rich noted, movement up thrust staircase trajectories create folds in the hanging wall, simply as a geometric response to the fault shape. These are fault-bend folds (Fig. 9.4B). Displacement on the thrust surface is passed beneath the fold. However, as thrusts lose displacement up-dip along a ramp, the resultant structure is termed a fault-propagation fold (Fig. 9.4C). These structures have distinctly steeper forelimbs than fault-bend folds. The up-dip termination of thrusts faults are called tips. Folds formed above a basal slip surface, without thrusts cutting their forelimbs, are termed detachment folds (Fig. 9.4D). In effect, these form in response to displacement gradients on thrust flats, rather than ramps as for fault-propagation folds.
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