The geometry, kinematics and rates of deformation within an en échelon normal fault segment boundary, central Italy

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The geometry, kinematics and rates of deformation have been investigated within an en échelon fault segment boundary between two left-stepping major normal faults in the central Apennines, Italy. Examination of faulted post-glacial sediments and geomorphic features attributed to glacial retreat (18 ka) reveals that the two major faults dominate the recent slip, with rates that are 5–10 times greater than the other faults. At the centres of the major faults, slip is parallel to the regional extension direction (NE–SW) defined by focal mechanisms and borehole breakouts. Slip-vector azimuths defined by striations on faults in the stepover zone indicate NE–SW extension on NW–SE faults, sub-parallel to the regional extension direction, together with significant and contemporaneous NW–SE extension, along the strike of the Apennines, on ESE–WNW and ENE–WSW faults. Thus, distributed three-dimensional strain occurs in the stepover between the two major bounding faults. Extension rates summed across the stepover are 0.68 mm/y parallel to the regional extension, compared to ∼2 mm/y at the centre of one of the major faults. Extension rates along-strike of the fault zone are 0.14 mm/y within the stepover and zero at the centres of the two major bounding faults. The above information is used to discuss how extension is partitioned between different structures during the growth and linkage of normal faults.